# automatic light system

## Hi, please how can I delay the LED for turning on, even if the transistors are already biased?

• ### will a RC time delay work?

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• ### thanks for helping

Votes: 1 100.0%

• Total voters
1
Thread Starter

#### antsoua

Joined Jan 5, 2015
3
Hi, please how can I delay the LED for turning on, even if the transistors are already biased?
for 10sec!

even if the

#### ericgibbs

Joined Jan 29, 2010
11,637
hi,
Are you asking how to modify the circuit to have a 10 sec turn On delay.??
E

Thread Starter

#### antsoua

Joined Jan 5, 2015
3
yes that's what i'm looking for!

#### InspectorGadget

Joined Nov 5, 2010
215
Well, 10 seconds from WHEN? It's pretty important to give the context of the circuit, when you're talking about a dynamic function like this. What's it used for? Why do you want a 10 second delay?

The decreasing light on the LDR causes its resistance to slowly increase. The decreasing current through Q1's base allows more current to flow from R3 into Q2's base. At some point Q2 first starts to turn on (when the voltage drop across R3 is low enough to bias Q2) but the the current through the LED still increases somewhat gradually after that (as the current through Q2's base increases) until the point at which Q2 is saturated.

I guess if the gains are high enough this whole process happens rapidly. But a capacitive delay in this kind of system will just draw out the gradual nature of the turn-on process and that may be what you're trying to avoid. So answering the questions at the beginning of my post will help establish what you really want to do.

#### Dodgydave

Joined Jun 22, 2012
9,870
Put a 100k resistor in series with Q1 base, and a 100uf capacitor across R2, this will slow the ldr response.

Thread Starter

#### antsoua

Joined Jan 5, 2015
3
Thanks Inspector Garget for the clarification,
Now suppose that this circuit is outside, and the sun begins to fall, the LDR gets flickering light! like fast on and off for a while, before becoming stable(light) !
I actually want to include a kind of “Power Start On-Delay” which takes effect as soon as the base of Q2 is biased, then hold the unsteady flow of electrons and release it after 10sec!
I assume after 10sec the amount of darkness will be enough to allow the LDR to stabilize the LED.

Ok Dodgydave, let me try it

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#### InspectorGadget

Joined Nov 5, 2010
215
Well, you could do it with a big time delay. But I think the simple R/C delay will just make the flickering slower. Depends on the character of the flickering. But dusk takes minutes to hours relative to a 10 second time constant.

Better to do it with hysteresis. Put a higher value resistor, like starting with 47K, from the base of Q1 to the collector of Q2. As soon as Q2 starts to conduct, it'll drive the base of Q1 even lower and then if the LDR decreases a little in resistance again, it'll take a lot more to induce Q2 to turn back on. Then if it still flickers a little, decrease the feedback resistor value. If the light won't turn back off in daylight, increase the feedback resistor.

#### Dodgydave

Joined Jun 22, 2012
9,870