How to use an LDR as a switch

[Rexlan]

When it is daylight I want the circuit to work normally. When it is dark I need the circuit to be disabled so I think I need to have a LDR in series with my logic line.

Help please!

**broken link removed**

 

[MikeMl]

What you dont want is a situation where the LDR is half way between light and dark where that biases the PFET into a state where it is half on/half off as that might cause excessive dissipation in the PFET. I would feed the LDR into a comparator (LM3xx type) to create Light/Dark logic level which is combined with the "logic Signal" in an AND gate. If "logic Signal" comes from a uController, then feed the LDR comparator into an input pin and do the "AND" in software.

 

[Rexlan]

Hi Mike ... I am physically at limit in the box so I am stuck with looking for a simple hardware solution. I was hoping that I could use the LDR with another transistor to either run the logic signal through to control Q1 or perhaps disabling it by lifting the ground?? (without destroying it).

This is the high side switch you gave me and I found a smaller footprint PFET that works quite good and can carry 6 amps.

 

[MikeMl]

What are the specs for the LDR? Dark resistance, Fully illuminated resistance? Resistance at the illumination level where you want it to enable the driver?

What is the nature of the load connected to the drain of the PFet?

 

[Rexlan]

I have not chosen an LDR yet so that is open ... cheap is the operative term for the spec ... LOL.

The load on the PFET will be about 6 amps and it is modulating a headlight. The logic out of the PIC is a 240 Hz PWM signal.

I had thought about trying to run the logic signal through a PNP with the LDR turning it on/off ????

 

[MikeMl]

Here is a 6A switch which turns the load ON only if the LDR resistance is ≤ 75KΩ. It is OFF whenever the Port pin is Low(diode). It has positive feedback so that the dissipation in the FET is minimized as the LDR slowly changes resistance.

 

[Rexlan]

Thanks again Mike ... I am on the road now and will get on this when I get home!

Mike I can not quite see the drawing on my laptop in the small .png file. Can youemail this to me please?

rbl671 at yahoo . com

 

[MikeMl]

Trying it again as a JPEG

 

[Rexlan]

Got it ... thanks.

You are very helpful Mike!

 

[Rexlan]

I miss-stated the PWM spec. It is 960 Hz at 60% duty cycle ... does that matter?

Mike can you recommend an LDR to use?

I plan to use the Fairchild FQU11P06TU FET. Is this OK with your circuit?

Thanks