Silicon Chip Online - Ask Silicon Chip

Pic-Toc clock questions

I would like to build your PicToc clock, as described in the July 2001 issue. When comparing the schematic with the PC board layout, a few things are not clear to me. Why are 4-pin pushbutton switches on the PC board, while the schematic only shows 2-pin switches? Are 2-pin switches sufficient?

I cannot follow the circuit logic: which pins on the switches should be closed when the switch is in neutral position (if any)? Pin 17 of the PIC is connected to pin 8 of DISP4 but not with pin 3 of this same DISP4, as it is shown on schematic. I don’t understand where the common pins of all four switches on the PC board are connected? (V. B., Rijeka, Croatia).

The pushbutton switches are single-pole momentary contact types. The four pins on the switches are necessary in this project because the top set are internally connected and so are the bottom set of pins. The interconnections then make links for the PC board connections so that they can straddle tracks. When the switch is pressed it makes contact between the top and bottom contacts.

The displays have two common pins (3 & 8). Only one needs to be connected as they are internally wired together in the display. DISP4 shows pins 3 & 8 connected because they must be connected on the PC board as the internal connection between pins 3 & 8 is used as a link.

FM receiver wanted for transmitter kit

I was wondering if you could tell me of any suitable receivers for the Oatley K222 FM stereo transmitter kit. I plan to use it for a wireless microphone but I need a receiver that will have a line level output so I can plug headphones into it to monitor the sound and a microphone level output to plug into the video camera.

I wouldn’t know how to modify an FM radio to have both levels of output so if you have any suggestions they would be much appreciated. I figured tapping into the signal before it’s amplified would do for the mic level, with the line level as per normal, but I wouldn’t know where the preamp signal would be. (D. W., via email).

Virtually any FM/AM radio with headphone outputs can be made to do the job. You would just need a voltage divider across each headphone output channel to derive the microphone signal. Each channel would require two resistors in the voltage divider, say 1kΩ and 22&Omega to deck (0V). The microphone signal would be taken across the 22Ω resistors in each channel.

Zapper needs a crook battery for testing

I bought the Battery Zapper Mk.II kit as featured in the May 2006 issue and have put it together. I do not have a bad or dead battery, so to test it I connected it to my good car battery. All the LED indicators function as detailed in your test section, however the output voltage sits at around 13V. According to your test section, the output voltage should be around 30V and reduce slowly as the battery "recovers".

Am I getting a reading of 13V because the battery I have connected the Zapper to is good? I can hear a low-level buzzing noise which I assume is the sound created by the inductors and implies that the unit is working. (S. P., via email).

Your relatively low voltage reading is probably due to the battery being "too good", with virtually no sulphating and hence a very low impedance for the zapping pulses. The low-level buzzing sound is a good sign and indicates that the zapping is probably working correctly, as you suggest.

No error in circuit

I noted an error in the Suppressed Zero Meter circuit on page 7 of the November 2006 issue. The +Vin is connected to the 7905’s GND pin and its input is connected to 0V. Could you please confirm this as I would like to build this one and not blow up anything? (R. N., via email).

The circuit is correct. The 7905 is a negative 5V regulator and therefore its input and output terminals must be negative with respect to its GND terminal.

Questions on the Mighty Midget

I was looking at the Mighty Midget 70W amplifier (SILICON CHIP, March 2002) and was thinking of building two for a stereo set-up. I am thinking of running both off a toroidal transformer with a suitable rectifier and filtering capacitors.

How much current does the amplifier draw and what is the maximum voltage? The toroidal is a 12-0-12V 160VA unit capable of 13.33A when the secondaries are wired in parallel. Can you suggest capacitors for filtering the power output?

Also, if both of the amplifier chips are installed on the same heatsink, do they have to be insulated from each other? (N. K., via email).

The TDA1562Q can accept a supply of up to 18V DC (maximum). The capacitors on the PC board should be rated at 25V if you give it more than 16V DC.

The 12VAC transformer may deliver more than 18VDC (after rectification and filtering) when unloaded. Make sure that this is not the case even with a high mains voltage. Ideally, the transformer should be rated at 10V AC rather than 12V AC.

The supply filter capacitors should be rated for a ripple current of at least 8A. Typically 4000μF 63V canister type electrolytics have a ripple rating of about 4A, so two or three of these would be suitable. Alternatively, use three or four 4700μF 25V PC-mount electrolytics. Use a 35A bridge rectifier.

The TDA1562Q amplifier package is isolated from its pins and so you do not need to insulate between them or the heatsink.

Re-using a notebook LCD as a monitor

I am enquiring about the possibility of utilising an old notebook’s LCD as a standalone monitor, say for a desktop computer, with a VGA input or even better, with an RCA plug input.

My actual use is for my multimedia player monitor so I can build an MP3 jukebox without having to use a notebook. (A. M., via email).

Many recent notebooks include an external video input but we cannot see how it would be economically viable to add a video input to those that don’t. LCD panels often employ custom-designed, low-voltage digital interfaces (ie, they are not analog compatible). This means that you’d need an LCD controller (with VGA input) capable of driving your particular model of LCD panel. Such devices are available but will probably cost more than the price of a cheap LCD monitor.

Noisy reception on AM car radio

My car radio gives me clear reception on FM but gives me noisy static with AM. Is there a way I can clear this noise or reduce it? (F. S., via email).

This is not a topic we have dealt with in a magazine article but there are a number of questions you need to resolve. First, if you are plagued with ignition noise and alternator whine while the engine is running, it suggests that you need suppression components installed to eliminate this problem. AM radios are usually more sensitive to this problem than FM radios. A car radio installer can fix this.

Second, if there is no engine noise but the sensitivity is poor, perhaps the antenna peaking capacitor for the radio has not been adjusted for optimum reception.

Finally, it must be said that most AM radio sections in today’s car systems are pretty poor and nowhere near as good as they were when the radios were designed and manufactured in Australia.

Priority switching for two amplifiers

I was wondering if there was a circuit that exists (or could be modified) to allow a pair of speakers to be connected to two amplifiers with one having an "override" on the signal. For example, the speakers would receive the signal from amplifier 2 but as soon as a signal was detected from amplifier 1, then 2 would be cut off and you would hear only what was broadcast on 1.

I know various PAs do this at an input level but I’ve never seen it done at the output end. (P. I., Carlton North, Vic).

You could achieve this "priority" function quite easily by modifying the VOX circuit we featured in the September 1994 issue. This monitored the signal from an electret microphone and it closed a relay whenever a signal above a certain threshold was received by the microphone.

You could adapt it by substituting a 10kΩ resistor for the microphone and couple the signal from your amplifier via a 1kΩ resistor into the 0.1μF capacitor feeding diodes D1 & D2. Make sure you disconnect the 0.1μF capacitor from pin 1 of IC1a.

You will need a DPDT relay to do the switching from one amplifier to the other.

Using the slave trigger with two flash units

I have successfully built the Slave Flash Trigger featured in the July 2003 issue.

Is there a simple way in which it can be made to trigger two electronic flashes at the same time? I was thinking of connecting two identical flash units to it by combining their trigger cables into one with a simple adaptor, or possibly incorporating a diode into each flash lead to stop them interfering electrically with each other. (E. J., via email).

You might be able to trigger two flash units with the Slave Flash Trigger using an isolating diode in each flash lead, as you suggest.

If this doesn’t work reliably, try fitting a second C106D SCR and diode bridge, to drive the second flash. The gate of the second SCR can be connected to the emitter of Q2, in parallel with the existing SCR’s gate.

No Support For ATtiny13 Or ATtiny2313

I recently built the AVR ISP serial programmer from the October 2002 issue of SILICON CHIP and I was curious to why it doesn’t support more devices like the ATtiny13 or ATtiny2313. Is there a way to program these other devices? (T. H., via email).

The ATtiny13 and ATtiny2313 were released well after the original AVR ISP project which is based on Atmel’s application note AVR910.

Several hobbyists have updated the AVR ISP firmware to support faster programming and some newer devices. You might like to try the V3.8b firmware update from this site: www.mikrocontroller-projekte.de (this is a German site, so use one of the on-line web page translators to access it). Note that to use this particular firmware update, you must adjust the crystal frequency and MOSI/MISO pin assignments to suit the programmer hardware.

Unfortunately, Atmel no longer officially supports the AVR ISP. This means that they’re not updating AVRprog to support newer devices. The latest version available is V1.40, as supplied with AVR Studio V4.11. This version should allow you to program the ATtiny13, once you’ve updated the firmware in the AVR ISP as described above.

Unfortunately, AVRprog V1.40 lacks support for the ATtiny2313. We’ve heard that the older V1.37 (as supplied with AVR Studio V3.56) does support the ATtiny2313, al-
though we haven’t tried it. Alternatively, if you don’t mind using a command-line interface, try AVRDude v5.1 or later from http://download.savannah.gnu.org/releases/avrdude.

Another option would be to purchase a genuine AVR-ISP MKII programmer, which supports all in-system programmable devices. You can get one of these from JED Microprocessors at www.jedmicro.com.au/avr.htm.

Bridging Studio 350 Amplifier Modules

I want to build an amplifier system into my JBL JRX 115 speaker, which has an 8Ω impedance and is rated at 250W RMS.

I note that your Studio 350 develops only 200W into 8Ω. Can I use two amplifiers in bridge mode to get more power? (B. L., via email).

Building such an amplifier system would be a major undertaking and would require significant audio-related electronics experience but yes, it is possible. As a bare minimum, you’d need a set-up like that shown in the accompanying block diagram.

As shown, a preamplifier boosts signal level ahead of a bridge adapter, which drives a pair of amplifiers via its complementary signal outputs.

Power for the preamplifier can be conveniently picked off from the bridge adapter’s ±16V supply, which is generated by on-board zener regulators fed from the amplifier’s DC power rails.

With the speaker "bridged" between the two amplifier outputs, you can expect about twice the rated 4-ohm output power, so for the Studio 350, this amounts to about 700W RMS. Note that as each amplifier "sees" only half the loudspeaker impedance, you can only use an 8-ohm speaker in this configuration.

Getting rid of the heat will be a challenge for a powered speaker application. We expect that the amplifier enclosure would need to be built on to the speaker, rather inside it. It’s also a very good idea to include speaker protection (not shown) for any high-powered amplifier. Without protection, amplifier failure can easily destroy an expensive speaker in seconds.

All modules are still available as kits. Check out Altronics for the preamp (K-5512), bridge adapter (K-5565) and amplifier (K-5815). Jaycar also stock the preamp (KC-5159) and amplifier (KC-5372).

VCRs For Security Systems

The article by Murray Downes on page 46 of the September 2006 issue initially looked like the answer to my prayers, as I want to use a video recorder on my home and at my weekender to record each time the system at either property was triggered. Unfortunately, though, this article didn’t solve the problem in my case.

The problem is using a video recorder as a security recorder. I’ve tried several methods of starting and stopping both Beta and VHS recorders, mainly using relays. However, I find that loading and unloading the tape each time the unit is activated is a waste of time and a potential cause of mechanical failure, even if the unit is left turned on permanently (waste of power also).

I’m now sorry I gave away my original piano key Sanyo Beta machine, as it may have filled the role.

Scots-byterians like me can’t see why security recorders or cards for computers should be so expensive. I’m sure an article describing how to disarm the loading/unloading procedure so that VCRs can be used for this purpose would be of value to a lot of readers (or am I out of step?). I have a number of Beta (mainly Sony) and VHS recorders that I would like to fire up to record when the surveillance cameras fire up. (J. P., Epping, NSW).

We do not see how you can hook up a VCR in any security system without using relays in the way you describe – that will naturally involve loading and unloading the tape each time an event is to be recorded. The only way to avoid that would be to modify the recorder so that the tape is always loaded (around the drum) but then you would presumably need a special tape-unloading procedure so that the cassette could be ejected from the machine when you want to do that.

Such a modification could be very tricky since the whole tape loading/unloading procedure is normally under microprocessor control.

Even if such a modification was successful, the VCR would still need to be permanently powered, otherwise short "events" might be missed. Hard disc video recorders get around this problem but they also need to be permanently powered.

Warning

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