dc mode to ac mode

[orkeet]

can anyone help me how to change this darkness detector circuit from dc to ac mode? asap please.. thanks

 

[audioguru]

Does your darkness flutter and make AC light and dark?
Mine is steady DC darkness or steady DC light.

 

[orkeet]

i want this circuit using ac voltage input and still can work as darkness detector. i've tried to replace the dc voltage with ac voltage but it seems like the circuit doesn't work as darkness detector anymore. i think the circuit needs some modifications but i can't figure out what it is. i still need two op amp in my circuit. i'm still new in this design thing.

 

[audioguru]

AC Input?
The input to your circuit is light or darkbness, not AC or DC.

Do you mean that you want to power the opamps with AC?
Opamps need a DC power supply or battery. It is easy to rectify and filter AC from a transformer to make DC.

Your circuit does not need two opamps. It will work with a single opamp if you replace the second opamp with a current-limiting resistor then the first opamp can drive the LED properly. Swap the inputs of the first opamp.

Your circuit does not need two batteries.

 

[Torben]

i want this circuit using ac voltage input and still can work as darkness detector. i've tried to replace the dc voltage with ac voltage but it seems like the circuit doesn't work as darkness detector anymore. i think the circuit needs some modifications but i can't figure out what it is. i still need two op amp in my circuit. i'm still new in this design thing.

You don't need to modify the circuit itself, you need to add another circuit (as audioguru said) to rectify your AC supply to DC which the circuit can use. That circuit will not work when directly powered by AC. It's a simple circuit but if you're working with mains voltage then it can be dangerous if you don't know what you're doing.

Is this a school project which says you must use an AC power source and two op-amps? I think two op-amps is overkill for this. A couple of days ago I made a light detector using one transistor and a couple of resistors, and a much better one using two transistors. It can be easily changed to be a dark detector.

What exactly are you trying to do? Why do you need AC power and why do you want two op-amps?


Torben

 

[h.d]

youonly need to add the suitable rectifier after the ac input to get the needded dc-voltage value,so you need only rectifier

 

[Torben]

youonly need to add the suitable rectifier after the ac input to get the needded dc-voltage value,so you need only rectifier

. . .if you want a supply voltage which is "DC" only in that it's all on the same side of zero. Unless you want it to look like a mountain range you'll also want one or more smoothing capacitors. And if the source AC is high (say, mains voltage) you'll also need a transformer to step it down to a usable voltage before rectifying it.


Torben

 

[orkeet]

yes. this is a school project. my lecturer wants a circuit that use two op amps or three transistors. after i showed him the circuit which use dc power source, then he wanted it in ac mode. i've thought about adding a rectifier but i just want your opinions that maybe there is other way to change this circuit to ac mode. so i guess i just have to add a rectifier to the circuit.
thank you very very much for your opinions

 

[Torben]

yes. this is a school project. my lecturer wants a circuit that use two op amps or three transistors. after i showed him the circuit which use dc power source, then he wanted it in ac mode. i've thought about adding a rectifier but i just want your opinions that maybe there is other way to change this circuit to ac mode. so i guess i just have to add a rectifier to the circuit.
thank you very very much for your opinions

Does it have to run from the mains (a wall plug) or can you use an AC stepdown adaptor? If you can use the adaptor, you need only the rectifier and some smoothing caps and maybe a regulator; if you have to be able to plug it straight into the wall with no adaptor you have to use a transformer too.

I'd go with the three-transistor option, although I think that might be one transistor more than you need (I'll keep thinking about that). Maybe you could use the third transistor and a zener diode to craft a simple regulator and the other two transistors to form a Darlington.


Torben