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I'm working on building a photosensitive alarm clock for a school project.

My supplies require me to use a DC power source, and my speaker requires an AC current. In the circuit I posted below, I attempted to do this in a two step process, where I first create a circuit that can vary its DC output based on light levels, then attempted to switch that to AC through an op-amp multivibrator.

enter image description here

This is the circuit I have come up with, but currently it doesn't work. For reference the top node is +12V and the bottom node is -12V. Do you guys see any glaring wrongs with what I have put together and any potential fixes that you could suggest?

Zinggy
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  • First, don't make us guess. So, define what you want it to do. Such as, switch the buzzer on in one state and off in the other. Or, make the buzzer pitch (or volume) vary with the light level. Then we can take it from there. For example, a diode between U? (oops, no reference designators. Another thing to fix) ... between the first opamp, and the R/C, will stop the oscillator in one state, and let it do its thing in the other. –  Dec 16 '20 at 18:34
  • Nice job on the circuit diagram layout. However it would help us to discuss the circuit if you numbered all the components. e.g. for the resistors R1-R7. What is the type of op-amp? Disconnect the output of the first op-amp from the multivibrator . Does it make a noise? When fault finding electronics it helps to isolate sections and see if each one is working. – RoyC Dec 16 '20 at 18:34
  • @BrianDrummond The circuits intention is to raise the volume of the speaker as more light is introduced in the circuit. With the way I have it set up now it *should* be in an "off" state while the photo transistor is dark, and be in an "on" state while the photo transistor is lit. – Zinggy Dec 16 '20 at 18:45
  • @RoyC I will attempt to do that, but with the software I'm using may be difficult to preform. op-amp is a LM324. When removing the input to the multi vibrator the speaker does not make a sound. – Zinggy Dec 16 '20 at 18:48
  • Why? You need to put component designations on your diagram. How long will it take to charge 1uF through a 1M resistor? – RoyC Dec 16 '20 at 18:54

1 Answers1

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  1. "top node is +12V and the bottom node is -12V" -> make sure, that both your op-amps powered in right polarity (it is not clear from your circuit - looks like second op-amp flipped vertically not proper)
  2. You are not separated multivibrator - the output of FIRST op-amp shorts your second op-amps feedback loop. You need to isolate it, and switch it ON by 1-st op-amp (which working like comparator), just second one select with EN (enable) pin option, and connect EN to output of the first op-amp.
  3. decrease 1M resistor, or decrease feedback capacitor. You have RC= 1000000*0.000001=1sek, so I suggest to use 0,1uF and 10k res to have RC=0.001 eq. frequency 1kHz

An example: Almost any n-p-n in this case will be OK. this case LM324 - both op-amps inside one case

enter image description here

Do not forget to add DC decoupling capacitor in serial with speaker - like on image 1...10uF

Second option: in this case QA2 should be in separate case (separate power connected by p-n-p). enter image description here

Pinelab
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  • For clarity, I am using a LM324 chip, which is four op-amps in one component. The chip itself is powered +12, -12, through Vcc and Vee respectively, I was just unsure how to convey that through a circuit diagram. This is something I feel I should know, but don't; how would I isolate the multivibrator in this circuit? – Zinggy Dec 16 '20 at 19:05
  • That's much better. – JRE Dec 16 '20 at 20:18