Basic circuit help please

RobH · 2020-01-29T20:48:11+00:00

I have put together a circuit from here: and this is the schematic: I have the led, ldr and the 100k pot part of the circuit working fine, but the buzzer from the 555ic does not work or sound as it is supposed to do. (the buzzer is new and and has worked on another project) I am using a BC327 pnp transistor as I don't have a BC557 transistor The author clearly has it working fine, but I cannot get it fully working. Hopefully as per the schematic, I have a wire from pin 2 on 555ic to the emitter of the BC327, and also a wire from pin 3 of the 555ic to the positive side of the buzzer. Now, the strange thing is after removing the said wire from the BC327 emitter, the buzzer does work, albeit when the ldr has not been seems to be due to the capacitor across pins 1 and 2 of the 555ic. Thanks

Jan 29, 2020 64 Replies

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Andy Bennet 6 years ago

Turn the 741 into a schmitt trigger, rather than trying to explain see:-

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amdx 6 years ago

Thank you, I did find a video showing that, but this page is much better. Mikek

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amdx 6 years ago

I may need a little more input, I have a voltage the sets at 5V, then drops to 2V. A negative going pulse. That is the 555 trigger on Pin 2. I need the output to go low when the input goes low.

Mikek

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amdx 6 years ago

positive swing on the schmitt trigger input to cause the negative output. Mikek

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Andy Bennet 6 years ago

Swop over the LDR and pot?

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