help with 555 schmitt trigger resetting

Hello I have built this basic sound to light 555 schmitt trigger circuit The circuit I am using (see fig.9) is at this link:

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The R1 & R2 are 220k, c2 is omitted,and its powered from a 12v car battery. Input to C1 is audio from an LM358AN that's fed from a microphone. The NE555 is driving 2 x TIP42A connected in parallel via a 150ohm 1 watt common base resistor,and the load is 2 x 24watt 12volt lamps also in parallel.

Here is my problem I need help with. Everything works fine but when there's no input sometimes the lamps remain lit, I need to get them to switch off automatically,but how? Thanks.

Reply to
zander
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That circuit switches the 555 output high with an input < 4V, and back low for an input > 8V. Where it ends up with no input just depends on the last excursion. It might not be the best circuit for what you're trying to do. If you change the value of R1 or R2 grossly, you could get the circuit to end up in the desired state, but it might be less sensitive. If the PNP output transistors turn on when the timer output is low, changing R2 to 100k while leaving R2 at 220k might leave them off most of the time by making the circuit usually triggered.

-- john

Reply to
John O'Flaherty

Whoops, I meant change R2 to 100k and leave R1 at 220k.

-- john

Reply to
John O'Flaherty

Thanks john, I'll try that. The TIP42's also drive a LED with 1k resistor, could the 10v on this resistor when the lights are on be used to somehow reset the trigger?

Reply to
zander

No, because unless you can delay that somehow, if the circuit can reset itself it will do so instantly. I'm not sure what output effect you are looking for- just a light on when there's an audio input? If so, you could detect the audio with an envelope detector (just a couple of caps, a diode, a resistor) and a comparator to compare the detected audio with some level. Then that could control the light. If the circuit is supposed to make the light flutter or something, you could use the envelope detector to gate the output of your existing circuit so it would turn off when there was no input.

-- john

Reply to
John O'Flaherty

The output effect is, the lights flash in time with the loadness of your voice. lowering R2 to 100k didn't work well, the lights did always go out but the effect was lost. I'd like to try a envelope detector to trigger the 555 so it turns off when there's no input. Being a novice I searched online for info on envelope detectors and found this link,

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but finding the values for C or R,looks complicated. Would using an IN4148 diode, 100n cap and a 10k resistor be ok to try? Does the diode connect to the existing C1?

-- Zander

Reply to
zander

The complication only arises if you are trying to recover a good audio waveform from modulated RF. For your purpose, you just need a long enough time constant (R*C) so you get a steady output when there's audio. Say you want it to respond in about 1/2 second. Then R*C should equal about 1/2. Starting with your 10k resistor, divide 1/2 by that, and you get .5/10000 = .00005, which is 50 uF. That's kind of big, so crank the 10k up to 1 Mohm, and use a .5uF cap. Then you'd have to use a comparator with input resistance a lot higher than 1 Mohm to not load it. Ayway, unless you're playing with line voltage, anything you want to try will be ok. Thinking about it, there's probably a simpler way than using an envelope detector and comparator. You could use another 555, triggering off the same signal, but use a one-shot that retriggers as long as it has an input, and doesn't reset until about 1/2 second after the last input. Then you can gate the inputs together so turning the light on requires both 555s to be triggered. There's probably a suitable circuit on that page you referred to. The new 555's output could hook to the reset pin on the first one, but the way you've got it hooked up, it might keep the lights on rather than off, so you'ld just have to rearrange the circuit so it worked right. Good luck.

-- john

Reply to
John O'Flaherty

I want to address something you did not ask about. You say you are using the TIP 42's in parallel. When you do that (use bipolar transistors in parallel), you need to use equalizing resistors - usually very low value, like .1 ohm - between each emitter and the supply. In this particular circuit, there is no need to parallel the TIP 42's. Just connect each of them to a bulb and don't parallel the bulbs. That way, you do not need the equalizing resistors. The bases can still be fed from the single 150 ohm resistor.

Ed

Reply to
ehsjr

The problem with paralleling junction transistors includes variations in the base emitter voltage for the same collector current, as well as various current gains. If one of these paralleled transistors (as you suggest) has a slightly lower base to emitter drop for the same base current, it will hog more than its share of the current from the 150 ohm resistor. The transistor with the highest base to emitter voltage will draw less than its share of the current. You would have to considerable overdrive the commoned bases to make sure the worst one saturated sufficiently.

If the collector loads can be separated as you suggest, you would do better (worst case saturation voltage for a given total base drive) by giving each transistor its own base current limiting resistor.

Reply to
John Popelish

Thanks! You're right - I overlooked that. It is better with separate base resistors.

Ed

Reply to
ehsjr

Feeding the audio through a 470k pot adjusting the input to a transistor inverter connected to C1 made a really big improvement. The pot sets the level that which the lights respond, and at maximum the lights don't flash at all.

And now when powering up the lights come on, then go off after a few seconds, even with no mic and the pot set to maximum. Its good this happens, just can't figure out why it does.

--
Zander
Reply to
zander

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