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Hi Everyone,

I have two circuits that work independently - an LDR controlled (Dark ON) circuit and a 5s Delay ON 555 circuit: -

https://www.circuitlab.com/circuit/955dh3/ldr-motor/

https://www.circuitlab.com/circuit/y3ufuz/5s-delay-on-timer/

I've been racking my brains trying to combine the two - when the LDR is covered, The 555 timer starts counting and when 5s is reached the output motor comes on and stays on as long as the LDR is covered. The circuit should then reset when light is sensed.

Any help would be greatly appreciated!

Many Thanks!

You might try a low-power relay in series with the LED in the LDR circuit, and have the normally open contacts break the trigger input on the 5s timer. When dark is sensed, the output of the LDR circuit goes high, and energizes the relay, thereby closing the contacts of the relay, and starting the timer.

Correction: Wire the relay in parallel with the LED!

Interesting problem.

Here's another possible solution: https://www.circuitlab.com/circuit/na3b2h/555_light_motor_rev2/

A few things to note:

1) I don't believe most 555s will run on 3 V.

2) With 3 V feeding the 555, the simulator shows 5 V output anyway.

3) The addition of a resistor in series with the LED as shown will limit current through that device to safe levels. More than about 20 mA is starting to push the limit and is not required for most applications.

Many thanks, very helpful!

Here's another possibility:

Simulate > Time Domain > Run Time-Domain Simulation

:)