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Possible to add a beeper to this circuit?

JT22

Nov 25, 2021
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Hello. I am currently writing about op-amps and 555 timers and found this circuit on this board. Is it possible that you can add an actual beeper circuit as well?

Any insight/opinion is greatly appreciated.
 

AnalogKid

Jun 10, 2015
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The circuit is not guaranteed to function, because the 741 is not rated to operate on 9 V.

Also, Q1, R6, and R8 form a very non-standard enable circuit. My guess is that this is necessary because of the severely reduced output voltage swing of the 741. Have you built this circuit, and does it actually work?

ak
 

JT22

Nov 25, 2021
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The circuit is not guaranteed to function, because the 741 is not rated to operate on 9 V.

Also, Q1, R6, and R8 form a very non-standard enable circuit. My guess is that this is necessary because of the severely reduced output voltage swing of the 741. Have you built this circuit, and does it actually work?

ak

I have built the circuit, but the LED does not toggle and it only stays on if a light or laser is shined on LDR 1.

When the light or laser is removed from the photocell the LED turns off.
 

AnalogKid

Jun 10, 2015
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Back to the beginning. Where did you get this circuit? Please post the document or a link to a site.

What circuit function are you are trying to achieve? To me it looks like an LED flasher circuit that is enabled when R1 has more light on it than R2.

What is your skill set for building this circuit on perf board or a solderless breadboard? Also, what other ICs do you have access to? LM358 / LM324? LM393 / LM339?

Where are you located?

ak
 

JT22

Nov 25, 2021
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I got this circuit from this message board and am trying to mimic what triumph is doing where the LED toggles in the end and wanted to see if you can add anything extra.

My skillset : this is my first circuit.
Location : Canada
 
Last edited:

Harald Kapp

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Nov 17, 2011
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The circuit seems to be a modification of this circuit with only one LDR.

The circuit is not guaranteed to function, because the 741 is not rated to operate on 9 V.
Right. The datasheet states under recommended operating conditions: supply voltage (min) +- 10 V.

In addition, assuming your specific chip would work from + 9 V, there is another limit, namely output voltage of the 741. The same datasheet tells us that the output voltage swing of the 741 is in worst case 3 V from the supply voltage, which in case 0f a single 9 V supply means the output swings between +3 V and + 6 V (assuming >= 10 kΩ load, which is given in this circuit). Adding 0.6 V base-emitter voltage of transistor Q1 means the voltage on the trigger pin 2 of the 555 timer ic would not go below 3.6 V (worst case, lower if ther output of the 741 is better than specified).
The trigger input (pin 2) of the 555 requires < 3 V when the 555 is operated from 9 V (datasheet). As the input voltage to pin 2 of the 555 will under the above conditions not go below 3 V, the 555 will never be triggered.

An LM358 as suggested by @AnalogKid is a much better choice instead of the 741. Notethat these chips have different pinouts, you'll have to adapt the pin numbering when using anoter opamp instead of the 741.
 
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