Sound to light for my son's Dalek

[trog]

At my wits end with this so hope someone here can give me some help:

The attached is a circuit I have made for my sons Dalek, it takes a voice input (Which is shared with a voice modulator) to flash his lights. I have an extra LED from the input to the MOSFETs to earth to confirm the ouput from the circuit. OK here we go with the problem:
Without any bulbs connected to the FETs the circuit seems to work, though VR2 which is supposed to change the "latch" time doesn't seem to do much. With a single 5W bulb it still seems to work, but only if VR2 is turned right down to almost 0. Anything above 0 at it just latches on with the first sound input and stays on forever. With over 10W (and VR2 turned to almost 0) things get even odder!:. At low sound input it seems to work OK (but dim) but at higher inputs it "latches on" with the first high sound input, and stays on until the next high sound input.

Changed/Checked components, rewired the FETs on a different circuit board... What's wrong? Please help, please.

 

[KrisBlueNZ]

Hi there and welcome to Electronics Point

That circuit is not the best, but it should work.

What is your power source? Is it rated to supply all of those lamps? Does the voltage drop when the lights are illuminated?

 

[trog]

Thanks.

Power Source is a 12V 9Ah lead acid battery (from my eldest sons sprint kart). Showing 12.5 voltsat the circuit board, drops to about 12.2 when the lights are on.

 

[KrisBlueNZ]

OK, well that should be fine!

Can you upload a photo of your construction?

 

[trog]

Oh OK, It's not very pretty!

The board has the voice modulator as well, the light to sound is top right, mic input is grey coax at the top, between the light to sound and the pre-amp for the voice modulator. the FETs are bottom right, the white wires carry the signal from the module to the FETs. The red & black wires top right go to an LED which is for the output signal from the circuit to the FETs.

 

[KrisBlueNZ]

That's OK, it looks fine.

In post #1 you said you have an LED from the "input" to the MOSFETs to earth. Do you mean you've connected an LED directly from the MOSFET gates to the 0V rail? If so, that will be causing the problem by limiting the gate voltage. Remove that LED. There's already an LED connected across the light bulbs.

Leave VR2 set half way during any testing.

If you still have problems, check your construction again. You can upload a photo of the underside and I'll check it, but you need to inspect it closely, looking for copper whiskers shorting adjacent tracks and incompletely removed copper shorting across track cuts. Use a continuity tester.

 

[trog]

Thanks for the help mate,

Tried removing the LED, no change! with VR2 in the middle the lights light up at the first sound and stay on...

I have spent a long time checking for shorts on the back of the board, I don't have a continuity checker!

Here is the back of the board, worse than the front.

 

[chopnhack]

Here is the back of the board, worse than the front.

That's quite good for as many parts as you have on your board! Best of luck working on a hovering Dalek when he asks for one!!!

 

[KrisBlueNZ]

I'm finding it pretty difficult to follow those photos. I think you'll need to do the checking yourself. Does your multimeter not have a continuity check function? You can use it on ohms range (continuity will show as less than 1Ω) but it's really handy to have a meter that will beep, so you don't have to take your eyes off the probes.

I don't see any connection to pin 4 of the LM358. Is it there but I can't see it in the photos?

Recheck your layout and check against the schematic for things that should be connected together but aren't, and check the stripboard for shorts between adjacent tracks and across track cuts.

If you don't find it that way I'll do a step by step checklist.

Edit: It's a sound-to-light circuit, not light-to-sound.

 

[trog]

Kris,

Thanks for the help, I have found on error around T2. Good news; it now has an effect on the flash duration, as it should. Bad news it hasn't cured the main issue. The attached is the veriboard schematic I worked from, this is the second error I have found (despite reviewing it many times), first was the break in the board between the top terminal of T1 and C1/C6.

Would you be able to have a look at the verboard schematic to the original schematic to see if there are any other errors I haven't seen?

 

[KrisBlueNZ]

OK, I don't see any problems with the layout. That doesn't mean there aren't any, of course!

But here's a step-by-step checklist you can try.

Disconnect the lamps for this testing, and set VR2 to around half way. Measure all of these voltages using a multimeter on the DC voltage range with the black probe connected to the 0V (earth) rail of the circuit.

The VCC rail is pin 8 of the LM358.

Measure voltages on the LM358. Pins 1, 2 and 3 should all be around half of the VCC supply voltage, and all within about 0.01V of each other. Pin 4 should be 0V. Pin 6 should be between 0.4V and 0.6V.

Pin 5 should be close to 0V when there is no sound at the microphone. When you talk into the microphone, it should increase; with loud input, it should reach at least 1V.

Q1's collector should be close to the VCC rail voltage while there's no sound at the microphone, and should drop down to near 0V when there is sound.

Q2's collector should be close to 0V while there's no sound, and should jump up close to VCC.

 

[trog]

Well somethings not quite right:

Pin1 = 9.47V
Pin2 = 9.55V
Pin3 = 9.57V
Pin4 = 3MV
Pin5 = 0.47V going up to 2.3V
Pin6 = 0.47V

Q1 = 12.47 dropping to approx 0
Q2 = 0 raising to approx 12.47

Output seems correct at Q2 collector (that's the pin going to the FET)

 

[KrisBlueNZ]

OK, the DC voltages on pins 1~3 are set by R1 and R12. I can't see any problem in that area on either side of the board, so you'll need to check it carefully yourself, but that's the place to start.

The voltage on pin 5 should be a lot lower than 0.47V when there's no sound. You could check the connection to that point.

Since the drive voltage to the MOSFETs looks right, perhaps there's a problem with the current flow in the drain circuit. When the MOSFETs turn ON, current will flow from the +12V supply, through the light bulbs, into the drains of the MOSFETs, through the MOSFETs, out of the source of the MOSFETs, through the 0V rail of the circuit and back to the negative return of the 12V supply. All of the connections in that path should be made with wire, otherwise the current could cause voltage drops that might affect the circuit's operation. The rest of the circuit can be "hung off" the appropriate points on that current loop. That part doesn't matter because it doesn't draw much current. But the main current path should be made with wire.

 

[trog]

Kris, Thanks again, I have been looking at the MOSFET side, I have even made a second circuit and both behave the same... I think tonight I will try a relay, driven off Q2 to see what that does.
I was wondering if the issue may be the Gate on the FETS not being able to drain to 0? I noticed in playing with the FET circuit that a 12 volt input on the gate activates the circuit, which then stays on, even when the gate is disconnected from the positive, untill the gate is actually earthed (so not like a relay that requires a constant current.) If so would a diode between Q2 and R10 make a difference, forcing the FET to earth through R11? Or am I talking nonsence? Or have I got the wrong FETs?

 

[KrisBlueNZ]

OK. When you try the relay, try it with and without the lamps. If there's a difference in behaviour, that implies there's something happening to the power supply voltage when the lamps are present.

The MOSFET gate voltage is pulled to 0V by R11. You could check that it's present, the right value, and connected properly.

Another possibility is that the power supply is dipping when the lamps initially turn on, when their resistance is low and they draw more current than they do when the filament is warm. You can test this theory by separating the circuit through the lamps and MOSFETs, and powering the rest of the circuit from a separate battery. If that turns out to be the problem, you can fix it by adding a diode and a large electrolytic, so the rest of the circuit has a stable supply to operate from.

 

[trog]

Some sucess:

Tried changing R11 to small values; no success!

Pins 1-3 are still up at about 8 I will keep working on that... (The modulator pre-amp is at 6, which is an identical circuit) Pin 5 is now down to 0.47mV at rest going up to 6ish Volts, after cleaning all the gaps between the tracks.

Q2 wouldn't drive a relay but when connected to a FET it worked (but looked rubbish as the flashes were too boolean!)

When I connected the FETs and circuit to separate batteries (common earth) it worked perfectly which is a bit of a surprise as the battery is big! Could you tell me what to do with the diode and capacitor, or would it be better to use a voltage regulator to drop the whole circuit to 9 volts? (Been thinking; with the whole modulator circuit working too when I talk it's working hard?)

 

[KrisBlueNZ]

OK. Replace R5 with a diode such as 1N4004, pointing leftwards (i.e. anode to battery positive, cathode to the small-signal circuitry). You may also need to increase C4. That should solve the problem, but if not, you could add a regulator in there. I recommend a low dropout regulator such as the LM2941. There are several requirements if you use it though. So let me know if the simple fix doesn't work.

 

[trog]

Just tried with a 1N4148 diode... No luck. What size capacitor should I go to?
That regulator looks scary!

 

[KrisBlueNZ]

I suggested a 1N4004, not a 1N4148. 1N4148 is only rated for 100~200 mA (depending on manufacturer) and is not a power diode.

Try 2200 µF for C4. But also check the topology of your connections, as I mentioned before. The whole path from the positive side of the power supply, through the bulbs, through the MOSFETs and back to the negative return of the power supply should use thick wires, and the rest of the circuit should be "hung off the side" of that path.

Yes the regulator is quite a hassle. I hope we won't need to use it.

 

[trog]

Kris, I am conceding defeat! This hasn't worked, it didn't even move the "threshold" it still works at 5W but not at 10W. I am going to try a car battery tomorrow, i.e. something big enough to ensure no voltage drop but if that doesn't work I have bought some LED lamps, they are a bit bright and don't have the "historesis" of a filament bulb but they work; it's a solution. In a few months, when I have finished everything, I am going to make a new modulator, at that time maybe you could suggest a better sound to light circuit.

Mean time thanks so much for all your help I really appreciate you spending the time to help me.