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enter image description here

here is the circuit. I want it to flash a LED when it's dark, and completely turn off when it's not dark. so a LED is connected to Q2, and R4 should be a very high value to save battery life.

My questions: 1, will this circuit work for my purpose?

2, how to improve it to suit my purpose?

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  • \$\begingroup\$ I may not be correct, but this seems like a LDR aka light based oscillator so it should work. Have you tried it? \$\endgroup\$ – Passerby Nov 28 '15 at 8:29
  • \$\begingroup\$ If the LED is connected with its cathode to Q2's collector and its anode to ground, I don't think it'll work because Q2's collector will swing between the LED's forward voltage (<3.5V, probably) and about 0.3V instead of between Vcc and 0.3V. I could simulate it and find out for sure, but then so could you and find out for yourself, without me doing your legwork. \$\endgroup\$ – EM Fields Nov 28 '15 at 8:52
  • \$\begingroup\$ You can use comparator for your purpose ? \$\endgroup\$ – Iamat8 Nov 28 '15 at 8:54
  • \$\begingroup\$ What's the LDR's resistance in light and in dark? \$\endgroup\$ – Daniel Tork Dec 28 '15 at 15:02
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It more than likely won't work, and it's not worth "improving" if you want something easy and you need something guaranteed to work, like this:

The plot is an LTspice simulator output for the schematic shown.

the yellow trace shows that as the input voltage to U1B increases, eventually it'll get past U1B's switching threshold and gate the astable multivibrator comprising U1A,B,and C ON, as shown by the green trace. U1D is used as an LED driver, and to keep the current from the 12 volt source low, a high efficiency LED should be used in conjunction with a high-resistance ballast.

The input to U1B was generated by V2 for the simulation, but in real life it would come from the junction of R4 and R5, a voltage divider connected across the 12 volt and ground rails, with R5 (the LDR) connected to ground.

That way, as the LDR's environment grew darker its resistance would increase, increasing the voltage into U1A until it went past U1A's switching threshold, which would start the flasher. Then, as it got lighter, the voltage into U1B would fall until, eventually, the flasher would stop flashing and the LED would turn OFF.

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  • \$\begingroup\$ The switch on and off threshold will change with temperature and supply voltage variation with this type of circuit. Whilst this may be suitable for a one-off hobby circuit it is probably not at all suitable for a production circuit where repeatable performance in varying conditions is expected. \$\endgroup\$ – Michael Karas Nov 28 '15 at 14:52
  • \$\begingroup\$ @MichaelKaras: Wow! Pretty snarky considering the spec's the OP posted and the circuit mine's replacing. BTW, what you're talking about seems to be the shift in switching point voltage with temp and supply voltage variations, yes? Do you happen to know what that might be or can you post a link to a source, please? \$\endgroup\$ – EM Fields Nov 28 '15 at 15:40

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