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Need help with school project; photocell, op amp, and 555 timer

triumph_013

Nov 19, 2015
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Nov 19, 2015
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I don't have a schematic yet, other than the one's I have already provided.

Ok, today I replaced the switch in the 555 diagram with a relay. I can put power directly to the relay and it toggles the LED just fine, but when I try to use the output from the op amp to power the relay, the op amp doesn't seem to work. I still have the LED in the op amp circuit to verify that it is working properly, but when I connect the output pin from the op amp to the coil on the relay, the op amp refuses to work. It starts working again when I disconnect it from the relay.

I will try to draw up a diagram after my next class to show you how I have it wired up.
 

BobK

Jan 5, 2010
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The op amp cannot provide enough current for the relay.

I would suggest configuring the 555 as a monostable and using the opamp to trigger it.

Bob
 

triumph_013

Nov 19, 2015
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Nov 19, 2015
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I guess I could do that, but that completely changes what I am doing. I really want to keep it as a bistable toggle if possible. Is there another way to get the op amp to trigger the circuit other than a relay?
 

AnalogKid

Jun 10, 2015
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Can I use a PNP transistor to drive the relay with input from the op amp?

Depends on the opamp. A 741 output can't get to less than about 1.5 Volts away from either rail. If you replace L1 with an NPN transistor with its emitter tied to GND, then its collector can drive a small relay. But you have to put a signal diode across the emitter-base to prevent reverse breakdown.

ak
 

triumph_013

Nov 19, 2015
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Nov 19, 2015
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I'm finally done, sort of. It is working, but the LED doesn't toggle until the laser STOPS touching the sensor. It toggles, and with momentary laser flashes it works, but with a long hold of the laser the LED doesn't toggle until the laser stops touching the sensor. Maybe it's how I have it wired up with the transistor?

Here is the diagram for how it is currently wired. The program I used didn't have a 741 op amp.

Screenshot%20112_zpslxn5az9a.png
 
Last edited:

GPG

Sep 18, 2015
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Sep 18, 2015
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Check the voltage at the opamp output under various light conditions including the laser.
 
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