Silicon Chip Online - Ask Silicon Chip

Confusion over ignition MAP sensor

I’ve recently started to build the Programmable Ignition System for cars (SILICON CHIP, March, April & May 2007) but I am stuck at the MAP sensors. The article states a Sensym Module ASDX015A24R is suitable device for non-turbo cars.

My problem is that when analysing the datasheets for the Sensym module it is not really clear if these devices will actually measure vacuum; they seem to specify only 0 to 15 psi not 0 to -15 psi? Would you mind commenting? (G. L., Mount Evelyn).

The 0-15 psi range relates to pressure referenced to a vacuum (0 psi). These sensors have two pressure ports but one is sealed off while the second port is open and measures the external pressure.

This type of sensor is called an "absolute pressure sensor" because they measure pressure with respect to a vacuum (0 psi). So a 15psi measurement is normal atmospheric (air) pressure (1013hPa or thereabouts) and 0 psi (0 hPa) is when measuring a vacuum.

The Sensym sensor specified is suited to measuring from 0-15 psi and that is the range from a vacuum through to atmospheric pressure. That makes them suited to normally-aspirated car engines where the manifold pressure varies from atmospheric through to reduced pressure. Those engines that have supercharger or turbocharger boost will require a sensor that can measure above atmospheric pressure, at say 2-bar (30 psi) or even 3-bar for higher boosted engines.

Relay Fault In The Studio Series Stereo Preamplifier

Some time ago, I bought a Studio Series Preamplifier kit. However, when I put it together and tried it for the first time, it didn’t work.

The fault is that the relays are not switching on any of the inputs. I managed to confirm that the rest of the preamplifier is working and then tried to find out what is wrong with the input switching. My findings are:

Q1-Q5 are soldered correctly.

The voltage on all transistor emitters is about +5V.

The base voltage of the selected transistor is about +0.96V and the collector voltage of the selected transistor is about +0.35V.

The base voltage of the unselected transistors is about 1.7-1.8V and the collector voltage of the selected transistors is about 1.13V.

If I put 5V of correct polarity across the relay diode, the relay energises as it should.

Could you please advise the correct base and collector voltages and give me a clue as to what is wrong and if it could be rectified? (H. B., via email).

The base and collector voltages for transistors Q1-Q5 are incorrect. If an input is unselected, the base of its transistor should be at +5V and the collector at 0V. When selected, the transistor base should be about +4.3V and the collector at about 5V to apply voltage across the relay coil.

Check that the correct transistors have been used, as they give voltages that suggest that they are either NPN transistors or have been incorrectly oriented on the PC board. Q1-Q5 should be PNP BC327 types.

Audio/video
switcher wanted

Has SILICON CHIP ever done a project whereby several units can be connected to a multi-controller which only has a couple of leads going to a TV? I have a VCR, DVD player and also a set-top box to connect up. (S. W., Grovely, Qld).

We described a 4-Channel A/V Selector in the April 2006 issue. This will switch composite video or S-video, together with L&R audio switching.

Errata for
Radio & Hobbies

What a wonderful resource the DVD-ROM of Radio, TV & Hobbies is. It is absolutely fascinating. I marvel at the commitment to scan all those magazines.

I am looking to build the "Twin 10 Watt Amplifier For Stereo" from January, 1959 and the "New Control Unit" from the May 1960 issue of Radio, TV & Hobbies. I haven’t been able to find any Notes or Errata on these projects by browsing the DVD and wondered if your records showed anything? (D. S., via email).

Back in those days there was very little published Errata and it certainly wasn’t archived in the way we do today for SILICON CHIP.

Having said that, we could not recommend that old amplifier and preamplifier. We have to say that all the old EA amplifiers were pretty mediocre by the standards of the day as compared to, say, the Mullard 5/20 or Quad valve designs.

Question On NiMH Charger

I wish to use an NiMH charger (SILICON CHIP, September 2007) to charge a 24V battery pack. However, the voltage required (36V) is above the specifications for this particular circuit.

I presume the voltage limit on this circuit is set by the maximum supply voltage of the LM358 op amp (32V). Is this correct? I realise I could use the earlier Power Tool Charge Controller (SILICON CHIP, December 2006) but this doesn’t have the current limiting features I require and would involve a more elaborate current limiting scheme.

Is it possible to supply the LM358 from the 5V regulated supply by re-routing the wiring to pin 8? Or can I substitute an op amp with a higher maximum supply voltage, such as the LM833? (S. H., via email).

The circuit could be used for the 24V battery pack if the supply rails to REG1 and IC2 are limited. The LM317 can handle 40V between input and output but a diode (1N4004) should be placed between input and output (anode to output) to prevent damage to the regulator if the input is shorted.

IC2 can be supplied from a zener diode limited supply with either a 10V zener diode in series with the pin 8 supply to give the required voltage drop from 36V to 26V or more conventionally with a series resistor and zener reference. A 20-25V zener would suit running at about 20mA, ie, use an 820W 1W resistor.

You cannot use an LM833 because the input does not operate at the ground (pin 4) supply. The LM358 can be used with an input at the ground supply.

Knock sensor
filter networks

I bought the Knock Sensor kit (SILICON CHIP, June 2007) which I plan to use independently of the Programmable Ignition System, as I am already running a Microtech Fuel and Ignition programmable ECU. I have assembled the Knock Sensor kit but in the instructions it tells me that the bandpass filter is set between 4.8-6.4kHz.

I have done a bit of research and apparently, based on my engine’s bore size of 81mm, the resonant knock frequency is around 7kHz (based on the formula: Resonant knock frequency = 900/(p x cylinder radius).

In the instructions for the kit, it tells me that the IC1b high-pass filter is governed by the 6.8nF capacitors and the 10kW and 2.2kW resistors. And the IC1a low-pass filter is governed by the corresponding 12nF and 3.3nF capacitors and the 5.6kW and 2.7kW resistors.

Could you please tell me if there is a formula that I can use to change the aforementioned resistor and capacitor values to adjust the high-pass and low-pass cut-off points? I would like the bandpass range to be roughly between 6.5kHz and 7.5kHz and if I know the formula I could experiment with the values to achieve the greatest accuracy. (R. T., via email).

The Knock Detector from the June 2007 issue was designed to be used in conjunction with the Programmable Ignition System. This is because the knock sensing circuit monitors the signal all of the time, while the Programmable Ignition board only checks the output from the Knock Sensor board around the firing point. This means that the Knock Detector may not be suitable for use with other ignition systems.

The formula for calculating the knock frequency does also depend on other factors apart from just the cylinder bore. It is also dependent on cylinder shape and the materials used in the engine. The 4.8-6.4kHz range for our detector was considered the best for most engines.

As far as adjusting the filter cut-off points, these are complex and require some component juggling. You could use the Spice programs located at the sites below. Choose the 2nd order Butterworth response and a gain of 1.

You need to set the frequency required and the program will calculate the values. The Active HP Filter site is for the high-pass filter, while The Active LP Filter site is for the low-pass filter.

http://www.beis.de/Elektronik/Filter/ActiveHPFilter.html

http://www.beis.de/Elektronik/Filter/ActiveLPFilter.html

Speed Controller For Xbox Fans

I have put together the 12-24V Motor Speed Controller (SILICON CHIP, June 1997) for the purpose of controlling the 12V thermo fans for my Xbox 360. At 12V, they are quite noisy so I just wanted to be able to adjust the speed of the fans accordingly. I have two problems.

(1) There is a high-pitched squeal coming from the motors. I have fitted the diode and capacitor before the fans and have tried 1mF, 0.22mF and 0.33mF capacitors as well but they didn’t help. What can I do to remove this noise as it defeats the purpose of the kit?

(2) I installed a larger pot to make it easier to adjust and so I could mount it where I needed it but when you turn it, the fans are at full power and I have only used maybe one third of the dial. Is there a way to make use of the whole dial, thereby making it easier to adjust? (J. B., via email).

The speed controller switches the power on and off at a fast rate to reduce power to the motor. With some motors, the windings or the laminated core will produce an audible tone at this switching frequency.

You may be able to reduce the noise by changing the frequency. You can vary the 10kW resistor at pin 6 of IC1 from 2kW to 500kW to get a change in frequency from 30Hz to about 10kHz. Use a 500kW trimpot (wiper connected to one end) with a series 2.2kW resistor to replace the 10kW resistor. Adjust this trimpot for minimum noise.

The control pot can be made to work over a wider range by adding a 4.7kW resistor in series at each end of the pot. So one end of the pot connects to ground (0V) via a 4.7kW resistor and the opposite end goes to +5V via a second 4.7kW resistor.

Adding a relay to Speaker Protector

Is your recently published Universal Speaker Protection & Muting Module (SILICON CHIP, July 2007) capable of providing current for two relays? Perhaps Q4 would only need the addition of a heatsink or would other modifications be necessary? I could imagine that someone with a multi-channel amplifier for home theatre or bi-amping would find it useful to run two relays.

In my case, I wish to use your module to remove turn-on thump in a headphone amplifier that is often connected directly to my CD player. The player is also used for the family stereo and rather than change leads when I want to use the headphones I wish to use a second relay with normally-closed contacts acting to bypass the headphone amplifier when power is off and to disconnect the bypass when power is on.

Perhaps if you could provide a figure for the current handling of the circuit, I could substitute a pair of lower power relays as one solution. (K. W., Newport, Vic).

To run two relays in parallel, change Q4 to a BC327 which has a higher current rating than the specified BC556. No other changes are required.

Speed control
for diesel shunter

I am intending to build a 5-inch gauge diesel shunter driven with a 12V 18A DC motor and want to use a speed control and drive it forwards and backwards. Do you have a kit that I can use? (B. K., via email).

Perhaps the most appropriate circuit would be the Reversible Speed Control in the April 2007 issue, rated at 12V or 24V and 20A.

Amplifiers And Hum Problems

I have two amplifiers which my Dad made and which look like Studio 200 versions judging by the MJ15003 and MJ15004 output transistors. He also had some old SILICON CHIP pages from 1987 & 1988 which describe the Studio 200 (February 1988) and what looks like the full article from the earlier version with other output transistors.

The latter text mentions using different Polyswitches for 4-ohm or 8-ohm loads. I was thinking of adding wires to the PC board terminals and up to a switch and then through either of the Polyswitches and back down to the board. The reason is that I am not sure what impedance speakers I might end up with. Can you see any problem with that or is there a more involved way to achieve the same objective?

Also, I would appreciate any tips you might have for reducing hum from my guitar amplifier. Any lead I plug into the input results in a lot of hum, increasing with the volume adjustment.

It is a brand new Roland guitar amplifier and it is not to do with the guitars that are plugged in. They are a Taylor Acoustic with Fishman pickup and a Les Paul with hum-bucker pickups. There are no single-coil "strat" guitars and not a fluoro for miles. I even turned off every circuit breaker except the one supplying the power point and unplugged every appliance on that circuit too.

The symptoms are: with nothing plugged in and volume up full, there is no hum; with lead and guitar plugged in, heaps of hum; with lead only, heaps of hum and with just an empty jack plug plugged in, heaps of hum. Touching the earth of the jack plug cancels the hum almost totally.

As is the way, I took it back to the shop and it didn’t do it in the shop. The shop guy said it must be relevant to my venue. I tried running an earth wire from the power point through a 50kW pot and on to the earth of the jack plug going to the input of the amplifier but no luck. I also tied one of those clip-on "dongle like things" to no avail.

Across the road is a large shopping centre, with the usual electrical gear and mobile phone towers on top. The electric train line is about 700 metres away. Because there is no noise with nothing plugged in, I’m guessing it’s RF noise, although the input jack might have a shorting arrangement internally. (L. K., via email).

We would be inclined to install Polyswitches for the 4W condition and leave it at that. In practice, how likely are you to seriously over-drive the speakers anyway?

As far as your hum problem is concerned, there appears to be something funny about the earth connection on your jack plug or the sockets themselves. Evidently, the earth side is not making good contact.

Flexible switching
for ceiling fan

I have a question regarding the connection of a ceiling fan. My electrician told me that it was not possible to connect the light for the ceiling fan into a 3-way switch.

Essentially the fan has a remote control that allows you to turn the light on and off. I want to be able to use a wall-mounted switch as well to turn the light on and off. I understand that the output from the remote control unit doesn’t have the ability to connect to the 3-way switch, as it is essentially a 2-way switch but would it be possible to do this using a relay? (A. F., via email).

The remote control to drive the light on or off probably uses a Triac to provide the switching of the lamp. This is therefore not suited to adding an external switch for 2-way switching and nor is it suitable for controlling a relay.

It may be easier to dispense with the remote control and just have a wall-mounted switch for the lamp or use two-hand held remotes, with one placed on the wall where the 2-way switch is required.

Variable Frequency Supply For Induction Motors

I have a number of of fractional horsepower induction motors collected from old home appliances and picked up at auction sales. However, they would be a lot more useful if I could vary their speed.

I know I can use universal (series) motors with an SCR speed control circuit but it is very hard to run something at a quiet constant speed under a varying load. My problem at the moment is a low-cost scroll saw with a 200W motor running at a constant speed. It would be much more versatile if I could run it fast or slow.

Would it be possible to build a 200W inverter with a variable frequency instead of a fixed 50Hz output? It wouldn’t need to be pure sinewave either. And rather than use a hefty transformer or a switchmode power supply to go from 12V or 24V DC up to 240VAC, could one not rectify the 240VAC mains AC to DC and then chop it back to AC that could be varied from say 20-100Hz? (K. C., Invercargill, NZ).

It is possible to make an inverter to run at a different frequency depending on its design. It is also possible to rectify and filter the mains and then regenerate a different mains frequency. You will find that the inverter rating will need to be much greater than the motor rating in order for it to start successfully.

Some ideas could be obtained from our 2kW Sinewave Inverter (October 1992 to February 1993) and the 200W Modified Square Wave Inverter from February 1994.

A question of
PC board layout

I am from the University of Western Australia Motorsport Team (UWAM) and I have a couple of questions you might be able to answer.

We are building an open-wheeled racer and are trying to put all of the circuits on as few boards as possible. Is there a problem with putting high-amp circuits near microprocessors and is there a way of avoiding said problem? Also, what are some good noise reduction techniques that we could employ to ensure the best quality signal, especially given that everything is constantly moving throughout the race? (Z. B., via email).

You can certainly have high currents flowing on a PC board with a micro on it. Just have a look at the 40A Speed Controller in the March & April 2008 issues of SILICON CHIP.

However, it does require careful layout of the PC board to ensure that the high currents do not flow in the signal sections of the microcontroller and that the high currents do not induce unwanted signal voltages into sensitive parts of the circuit.

Remote control fault
in preamplifier

I have built the Studio Series Preamplifier and all is good except I cannot get the IR remote control to work. The micro is working since the dry contact inputs work and the unit steps through the LEDs when in set-up mode.

I have put a CRO on pin 2 of the processor and the demodulated IR pulses are present and at a good level. I have used the exact remote as described in the April 2006 article.

I am beginning to think that the crystal is not running at 4MHz however I can’t get a conclusive reading off the crystal (pins 4 & 5). Any ideas of how I can figure this out from here? (M. S., via email).

There are several remote controls listed in the April 2006 issue: AIFA Y2E, RA7 and BC3000. Use set-up code 191 for TV. Make sure that the remote is encoded with this number correctly and that the TV button is pressed to select TV on the remote. Then with JP1 in place in the receiver, press one of the remote’s numeric keys twice. On the first press, the receiver’s ACK (acknowledge) LED should flash once and after the second press it should flash five times.

If this doesn’t happen then the remote has not been received correctly and the receiver will not respond.

If the crystal is incorrect in its frequency, the LEDs will not respond in the set-up. Crystal frequency measurement must be made with a high-
impedance low-capacitance probe. Use an oscilloscope probe on a 10:1 setting.

Additions to relay selector circuit

I found a "Circuit Notebook" item in the January 2006 edition for a pushbutton relay selector. Could this circuit be modified so that it does not turn one relay off before activating the next (ie, so that it gives the option of multiple "lives"?). (D. M., Camden, NSW).

In order to modify this circuit to allow more than one relay to be activated at a time, you’d need to use diodes (ie, 1N4148) to drive the relay driver transistors from more than one output of IC2.

For example, in order to have both relay 1 and relay 2 on together, you’d need to have an additional selector pushbutton switch connected between (say) IC2’s output 5 (pin 1) and pin 9 of IC1c, to create a fifth (1+2) selection setting. Then you would also use a pair of 1N4148 diodes to connect between the O5 output of IC2 and the gates of Q1 and Q2, with a 100W resistor in series with each diode.

Then when you pressed S6, the extra selector button, the counter would stop at the O5 position and both relay drivers Q1 and Q2 would be turned on together.

Testing a
CDI system

Is it possible to check a capacitor discharge ignition for faults with just a multimeter? (K. W., Maryborough, Vic).

Yes, it is possible. However, be careful not to touch the CDI except with the multimeter probes.

With the CDI powered up, test the high voltage supply which should be at around 300VDC if it has an inverter (ie, similar to our Multi-Spark CDI described in the September 1997 issue). If the high voltage is from a coil in the generator (see SILICON CHIP, May 2008), you should get some voltage when the motor is turned over. A typical figure would be about 50VDC on a multimeter.

You should then discharge the capacitor, using a 1MW resistor connected with insulated probes to the capacitor leads. Once discharged, check the voltage again before touching the CDI.

Next, check the SCR. Measure between the anode and cathode for a resistance greater than 100kW. Applying +5V to the gate input via a 100W resistor (and the 0V supply to ground) should cause the anode-to-cathode resistance to drop to a low resistance (less than 10W).

If all tests prove OK, then the CDI should work.

Attenuator wanted for Minispot oscillator

Shouldn’t the Minispot oscillator (SILICON CHIP, January 2008) have some way of attenuating the output level, so as to not overload the input to the IF stage, causing AGC to cut in and incorrect alignment to occur?

Would you use a pot? If so, what size and how should it be connected? We are talking about a high impedance circuit. (J. L., via email).

The Minispot 455kHz Modulated Oscillator was intended as a very low cost and simple circuit which can be used to align the IF stages of an AM radio. As the circuit stands, you can "attenuate" its RF output simply by moving it further away from your radio and the level will fall in proportion to the square of the distance between it and the receiver.

There is no easy way to add an attenuator.

Parts for
Prescaler project

I am trying to gather together all of the components for the prescaler project designed by Jim Rowe (October 2006). The ADCH-80 A wideband RF choke and the MC10EL35 JK flipflop do not seem to be readily available.

Do you have any suggestions for sources of supply or alternative parts? (N. M., Mt Pleasant, Qld).

You should be able to obtain the ADCH-80A wideband RF choke from the Australian representatives for Mini Circuits, Clarke & Severn Electronics. Phone (02) 9482 1944 or www.clarke.com.au

The MC10EL35 and other On Semi-
conductor ECL devices should be available from Future Electronics Phone (07) 3886 4933 or www.futureelectronics.com

How to record
teletext sub-titles

When watching many prime-time TV programs I prefer to mute the sound and rely on the subtitles available on Teletext page 801. This is because I often cannot distinguish the words from the foreground music. But I cannot find any way of recording the program to retain these subtitles.

I tried a set-top box which claimed compatibility with Teletext and it was OK for direct reception but not for recording. I also tried recording from the "TV out" sockets of a TV set which had this feature but again with no success. I asked several suppliers but the most useful answer I got was: "that’s a good question". Can anyone do better? (R. S., Armidale, NSW).

We put your question to Alan Hughes, who wrote the recent series of on digital TV. This is his answer:

No videocassette machine is capable of recording teletext subtitles in their undecoded form. The reason the VCR cannot record teletext is that the luminance channel of a VCR is up to 3.1MHz where as the teletext needs the full 5MHz.

The solution is to use a Personal Video Recorder (PVR) which will only record digital TV broadcasts. So in Armidale, NSW, you will be able to record all stations except NBN (but all stations in Armadale, WA). The PVR will have a choice on whether to record the subtitles and on playback you can choose to watch them or not. Check the menus in the shop . It will be in the installation menu. Then try it.

Notes & Errata

ds/PIC Programmer, May 2008: in the May 2008 issue (p67) describing the dsPIC/PIC Programmer, we explained that the MCLR-bar/V_PP line was deliberately switched between 0V and +13V. This was done to avoid possible damage to the microcontroller when it is in the ZIF socket.

However, if you wish to use the external programming header (CON3) with a microcontroller on a breadboard, for example, you should connect pin 1 of CON3 (the MCLR-bar/V_PP line) as shown in the accompanying diagram, adding a resistor (R) and diode (D) to the breadboard.

The diode and resistor allow the microcontroller to run when the MCLR-bar/V_PP line from the programmer is at 0V. There will be no possibility of damaging the microcontroller in this case as the microcontroller is accessed outside the ZIF socket. Hence the diode and resistor allow the microcontroller’s MCLR input pin to switch between V_DD and V_PP. This is more convenient while still developing the software.

The PGC, PGD and GND lines can be connected directly to the pins on the microcontroller, as explained in the article.

WARNING!

SILICON CHIP magazine regularly describes projects which employ a mains power supply or produce high voltage. All such projects should be considered dangerous or even lethal if not used safely.

Readers are warned that high voltage wiring should be carried out according to the instructions in the articles. When working on these projects use extreme care to ensure that you do not accidentally come into contact with mains AC voltages or high voltage DC. If you are not confident about working with projects employing mains voltages or other high voltages, you are advised not to attempt work on them. Silicon Chip Publications Pty Ltd disclaims any liability for damages should anyone be killed or injured while working on a project or circuit described in any issue of SILICON CHIP magazine. Devices or circuits described in SILICON CHIP may be covered by patents. SILICON CHIP disclaims any liability for the infringement of such patents by the manufacturing or selling of any such equipment. SILICON CHIP also disclaims any liability for projects which are used in such a way as to infringe relevant government regulations and by-laws.

Advertisers are warned that they are responsible for the content of all advertisements and that they must conform to the Trade Practices Act 1974 or as subsequently amended and to any governmental regulations which are applicable.

Confusion over ignition MAP sensor

Relay Fault In The Studio Series Stereo Preamplifier

Audio/video switcher wanted

Errata for Radio & Hobbies