50MHz frequency meter display freezes

I have recently built the 50MHz Frequency Meter (SILICON CHIP, October 2003) from a Dick Smith kit and have two problems.

(1) Initially the LCD froze and would only display a dash and I had to reduce the supply voltage from +5V to +4V to get the display to work. What has happened here?

(2) After power-up the LCD appears locked on "1Hz" and adjusting VR1 with the input shorted does not make it show "0Hz". The meter does not respond to an input signal. I can trace an input signal to pin 4 of IC1 but pin 5 does not show any signal, just +2.8V DC. How do I troubleshoot this problem?

At Royal Prince Alfred Hospital we have a Cyclotron that uses 11kW of RF power at 27MHz. I am hoping to fit a small antenna to this frequency meter and use it to show when RF power is "live". Any advice to get it going would be much appreciated.

Congratulations on an excellent magazine with the best DIY projects.(R. F., Camperdown, NSW).

Freezing of the display can be cured by changing the 470pF capacitor at pin 6 to ground on the LCD to a 2.2nF capacitor (code 2n2 or 222).

The 10116 (IC1) needs a 5V supply and so its operation should be restored when you increase the supply back to 5V. A sniffer coil made up of 20-30 turns about 10mm in diameter can be used to detect the RF signal. Connect the coil to the frequency meter input via screened cable (75Ω TV coax would be suitable).

Fooling the car’s ECU What do you think about the IAT resistors you can put in your car’s air inlet manifold? Are they rubbish or a worthwhile investment? They are claimed to trick your engine into thinking it is under a heavier load, therefore more BHP and torque will result as it opens the injectors slightly more. Are there any known problems you know of? (A. M., Adelaide, SA.) IAT stands for intake air temperature and the IAT sensor is a thermistor which drops its resistance as the air temperature goes up. If you substitute or add resistors to the IAT sensor, it will trick your car’s ECU into thinking that the air temperature is warmer than it is. Depending on the actual engine load and temperature this might not be good at all. We wouldn’t do it without a lot more research.

Troubleshooting a speed controller

About one year ago I built the Universal Motor Speed Controller (SILICON CHIP, September 1992) and it worked well. It was particularly useful with things like angle grinders when used with a wire brush. At the unregulated speed it throws out small pieces of wire from the brush that can really hurt!

Recently I went to use it and the fuse was blown and it blows any 10A replacement fuse that I put in it. I don’t know what could be wrong and I don’t want to replace parts one at a time, particularly as the project involves 240VAC. I have checked the wiring for shorted leads. Where do I start? (P. B., via email).

It seems likely that diode D3 has blown. Check with your multimeter to see if it is a short circuit in both directions. There is also a chance that the Triac has blown too. Again, check it with your multimeter between its A2 and A1 terminals. If they’ve both failed, it’s possible they have also taken out the SBS (Silicon Bilateral Switch).

By the way, angle grinders should not be used with wire brushes – they run much too fast, as you have found.

Digital speedo wanted

Is it possible to alter the Car Speed Alarm so it works as a stand-alone digital speedo? I realise we need to keep the Cal. mode but how do I remove the alarm section so it will still work? (R. N., via email).

As it stands, the Speed Alert can be used as a speedometer since it has this function. The alarm can be disabled simply by setting the alarm speed at the maximum setting.

Wrong rails from bench power supply kit

I recently purchased the Multi-Volt Bench Power Supply from the April 2002 issue. After construction, I found that the -3V rail was -3.7V and the -9V rail was -4.8V; all the rest were the correct voltage.

What is the cause of this problem? (S. C., via email).

You have an unusual problem with your power supply, if only those two voltages are incorrect. We haven’t heard of that one before and it’s hard to suggest what could be wrong.

All we can suggest is that you may have a couple of the resistors swapped – one of them for the 3V position of S3 and the 9V position of S4.

It’s hard to see what else could cause these faults, apart from perhaps faulty rotary switches or switches with the stop plate in the wrong position.

Joystick control of car windows

I am undertaking a project using your DC motor speed controller circuit from June 1997. I need to use a single control circuit or device (joystick/trimpot, etc) to vary both speed and direction of a car window motor. Can you help? (L. W., via email).

We suggest you look at the Train Controller in the April 1997 issue of SILICON CHIP. This provides speed and direction control with a single pot.

Battery Guardian disconnects fridge

I recently built a 12V Battery Guardian from the May 2002 issue. In my vehicle, I have two batteries which are isolated from each other when the ignition is turned off; it is a fairly typical set up in 4WD vehicles. One battery looks after the normal vehicle electrics while the second battery runs a fridge, lights, etc.

As the second battery is not used to start the engine, I set the Guardian’s turn-off point at 10.75V and the turn-on point was left at 12.5V. This works fine when running lights, etc on the output (ie, items which draw a constant current) but I have a 39-litre ENGEL brand fridge in the vehicle which draws 3.5A when running. This fridge creates a problem as follows.

With the battery voltage at 12.5V, when I turn on the fridge, the Battery Guardian immediately turns off and starts beeping. If I leave the fridge turned on and start the engine to raise the battery voltage above 12.5V, the Battery Guardian reconnects the fridge, stops beeping and the fridge runs, so the engine can now be turned off. The fridge continues to run till such time as the correct temperature is reached and the compressor in the fridge turns off.

As the temperature in the fridge rises, the compressor turns on and starts the above cycle again; ie, the Battery Guardian disconnects the battery and starts beeping.

I assume that this is caused by the battery voltage dropping below 10.75V for an instant as the compressor is starting or the current surge during start up is causing the problem. I thought that the Guardian would leave the battery connected for a short period as the compressor starts rather than disconnect the battery instantly. Do you have any suggestions? (B. C., Carina, Qld).

The Battery Guardian should work reliably even though the fridge draws more current during start-up. Your battery is probably old and on its way out. It may have a high impedance cell that drops rapidly in voltage with load. Have the battery checked.

Changing the slope on a sawtooth

I need to generate a non-alternating sawtooth waveform where the "up" section is very steep (ie, fast) and the "down" portion slower.

Many sawtooth examples I’ve seen, and indeed most circuit diagrams, demonstrate a sawtooth where the incline is slow and the decay rapid. I need the opposite!

I was wondering whether you could suggest a method of constructing this? Also, I had the idea of taking a simple sawtooth generator and simply rectifying it; in a sense "reflecting" the negative wave so it became positive. Would that work? (M. W., via email).

As a general rule, if you want different slopes on a sawtooth oscillator, you arrange for separate charge and discharge paths to the capacitor in the RC network. In practice, you can do this with diodes, one for the charge resistor and a separate one in the opposite direction for the discharge resistor.

Alternatively, if you have a sawtooth oscillator and you want to change its charge and discharge slopes, you could feed its signal through an inverting amplifier. This is the same as your "reflecting" idea.

Programming the Fuel Mixture Display chip I’ve downloaded your software (Air Fuel Mixture Display; September/October 2003) to program the PIC16F84 in .asm and when I’m trying to program it on my PIC, MPLAB (Microchip program) said some errors like: Error [118] D:\AIRFUEL.ASM 627: Overwriting previous address contents (0282). Warning [207] D:\AIRFUEL.ASM 1024: Found label after column 1. (clrw). Error [113] D:\AIRFUEL.ASM 1029: Symbol not previously defined Error [113] D:\AIRFUEL.ASM 1041: Symbol not previously defined (z). Can you help me resolve this problem please? I would like so much to make a Fuel Mixture Display for my car. (F. R., Rawdon, Canada). The microcontroller program for this project is also available for download as a ".HEX" file, suitable for use with your PIC programmer without modification. It is not necessary to assemble the ".ASM" file unless you specifically want to modify the original code. To get the AIRFUEL.ASM file to assemble successfully in MPLAB, you need to disable case sensitivity. This setting is to be found under "Project -> Build Options" on the "MPASM Assembler" tab. Note that you will still get a number of "Register in operand not in bank 0. Ensure that bank bits are correct" messages. These are just warning messages and can be ignored.

PWM train controller wanted

I am looking for a PWM (pulse width modulation) circuit for model trains. I used an old "Electronics Australia" project but mislaid the diagram. Its output stage used 2N3055s and the most important bit I cannot remember is the final stage crowbar circuit that protects the system using an SCR to inject a signal into the circuit to shut down the supply and illuminate a lamp. This is reset via a push-button. I cannot afford the fancy new DCC (Digital Command Control) circuits.

I have both books by Roger Amos ("Practical Electronics For Model Railways") but he fails to incorporate any simple overload protection using the SCR method. Can you help? I am sure their would be lots of modellers out there wanting a similar circuit for model trains. (D. R., Ruatapu, NZ).

We have published quite a few PWM train controllers over the years. The most recent are the deluxe Railpower published in October/November/December 1999 and the Li’l Pulser in the February 2001 issue. All our PWM designs incorporate overload protection.

Remote Control Extender Questions

I’ve recently purchased a Remote Control Extender kit (SILICON CHIP, July 1996) and was wondering if the following is possible. I would like to hook up two or three separate rooms to be able to receive remote control signals and have the transmitted signal sent back to a single destination using the one kit (if possible).

I’ve searched your site and noticed people asking for multiple IR transmitters (IR LED1) but not multiple IR receivers (IC1).

I was thinking I could house the PC board main unit near the receiving devices (VCR/DVD and A/V receiver) and use a single 9V DC power supply to drive the multiple IR receivers (IC1) rather than having to plug in up to three separate power supplies with three separate kits.

I was thinking I could do this by running three wires (using Cat 5e) in series to each of the IR receiver positions in the rooms but have no idea about the resistors, etc needed. Alternatively, if I used four wires, I could possibly accompany each of the IR receivers with its own acknowledge LED.

Please let me know if this is possible and how I could achieve it. (P. R., via email).

There is no reason why three IR receivers (IC1) cannot be paralleled, with the outputs connected together. The outputs are essentially open-collectors requiring a pull-up resistor.

In the circuit, they use a 1kΩ resistor in series with a LED. So use the one IR extender and add in two more IR receivers by paralleling the positive supply, 0V and outputs.

Note that each external IR detector would need a 10μF electrolytic capacitor across its power supply.

USB Ports For Older Computers

A friend and I have several older computers with USB header pin connectors on the motherboards. We wish to be able to hook up USB ports onto these computers but lack wiring diagrams for the motherboards. We wonder if you could design a device to plug into the USB port to test that the port is wired correctly and working.

The device would need to be undamaged by an incorrectly wired port and be incapable of damaging the motherboard if the port is incorrectly wired. I would imagine that a hardware/software solution would be needed, as a hardware only solution probably isn’t possible.

We realise that the simple solution is to purchase USB2 cards but the number of computers involved and the amount of use of the USB makes the cost prohibitive. Can you can come up with a solution? (B. P., Maryborough, Qld).

In almost all cases, information on motherboard USB header pin-outs can be tracked down on the Internet. In addition, the +5V and GND connections can be quickly verified with a multimeter, thereby preventing the serious damage that could occur if these two are reversed.

Your idea sounds interesting but as you suggest, many users are opting for USB2 upgrades on older systems. The cost of USB2 cards is now probably less than any project we could develop.

Connecting The High-Energy Ignition

I have just a couple of dumb questions regarding the kit "Universal High Energy Ignition" published in the June 1988 issue. I’ve built the points version of the kit.

First and probably the dumbest question is where is the positive side of the points? I can find the chassis side easily enough. Do I have to cut into some wires (like the main cable coming from the coil centre to the distributor)?

The other stupid question is this: if the centre pin of Q1 (BUP941) is connected to the coil negative, why all the effort to isolate Q1’s back tab from the case?

The construction side of the kit is fairly straightforward; it’s just the connection info that’s leaving me stumped. (S. S., Seaford, Vic).

The positive side of the points goes to the negative side of the coil in your existing set up. So take the negative wire off the coil – it becomes the positive points wire. You then run an additional wire from the coil negative to the collector of Q1. Do not touch the distributor HT wire from the coil – it stays as is.

Q1’s back tab is internally connected to its collector pin and it has high voltage on it as it switches. If you touch it while it is operating you will get a hell of a kick from it. So it must be insulated from the case.

By the way, we updated that project in the June 1998 issue of SILICON CHIP and then re-published it in "Electronic Projects for Cars, Vol 2".

Notes & Errata Micropower Battery Protector, July 2004: The article stated that the MAX8212CPA voltage monitor (IC1) is available from Farnell. Unfortunately, Farnell has discontinued this part. Wiltronics currently have the part listed in their catalog. Check their website at www.wiltronics.com.au or phone 1800 067 674 for availability. Alternatively, you can order a kit of parts for the project from Dick Smith Electronics, Cat. K-3132. SILICON CHIP has also recently become aware of a number of "bogus" MAX8212CPA chips that are being sold on the "grey" market. These devices are easily identified in-circuit because they behave in an opposite fashion to a properly functioning device. In "bogus" devices, the output N-channel FET turns on when the voltage applied to the THRESH input (pin 3) is less than the internal reference voltage. This causes the MOSFET (Q1) to turn on when the input voltage is below the set point, rather than above this point. 3V to 9V DC-DC Converter, March 2004: when the trickle charge circuit (D2 & R4) is installed and the converter is powered from the plugpack input without a battery connected, the output voltage will fall short of 9V. This occurs because the trickle-charge circuit is pulling the "SW" pin higher than the "VIN2" pin, causing the TL499A to erroneously select the step-up switching regulator instead of the linear regulator. If you must operate the unit from a plugpack without a battery installed, then you can solve this problem two ways: (1). Remove the trickle charge components (D2 & R4), or (2). Fit a 2-pin header so that the trickle-charge circuit can be disabled (via a jumper shunt) at will. You will note on the circuit board layout that provision has been made for this directly below D2. First, remove D2 and cut the small track that joins the two square pads. Install a 2-pin header and refit D2, noting that you’ll probably need a new diode with longer leads so that it can be positioned between the new header and L1. 50W DC Electronic Load, September 2002: the circuit diagram (Fig.2) shows the input pin numbers for IC1b in reverse. The non-inverting input should be pin 5 and the inverting input pin 6.

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.