Ambient control of 555 dimmer

[ZOR100]

Hello,

I am using a 555 to control the output from Infrared LEDS for home security lighting. It works well, however I have tried using an LDR and transistor to supply the voltage to the 555 in attempt to include a further automatic adjustment to the LEDS output dependent on ambient light level.

I could not get anywhere with it as the output of the transitor (taken at collector) was only ever half the supply voltage and this dropped further when loading further LEDS.

I was going to use an LDR in the leg of a voltage stabilizer to achieve a similar varying of voltage to the 555 circuit but thought I would ask for any advice. Thanks

Sorry, cannot send an attachment jpg??

 

[Reloadron]

It sounds like you are after using a LDR to generate PWM that will control LED brightness? Should that be the case I would think about using a circuit like this one. Rather than a 555 approach I would use the triangle waveform approach of PWM. The output of the linked circuit could be used to drive a MOSFET that would drive your LED banks. I would use a Logic Level Mosfet as the MOSFET.

Ron

 

[ZOR100]

Thank you Ron.

Maybe I went down the wrong road. Unfortunately I have put the 555 circuit onto a board with other bits, and was thinking it would be easy to provide a LDR referenced supply voltage to the 555 circuit, reproducing what I had seen to work when turning down my power supply. Thats why my other thought was having an LDR in the ground pin of a voltage regulator (eg 78120)

Basically I was using an LDR and a resistor as a voltage divider which went into the base of an NPN TIP 50 transistor. The emitter was taken to 0v. The collector had a resistor to +ve, and was used to supply the 555 and its transistor (driven from pin 3 of the 555) which fed the LEDS. The resistor in the collector to +ve15 volts ended up being 10 ohms in attempt to try and get the highest voltage to the LEDS. I had no resistor going from the centre of the potential divider to the base of the transistor (in my LDR part) as it reduced my output voltage slightly. So basically I was having nothing more to remove in my circuit to get more voltage/current output. Maybe the transistor is not correct for this?

Thanks again

 

[Reloadron]

I wish you could post a schematic. When a 555 is used as an LED dimmer circuit it is generally a design like this one. It sounds to me like you are using the LDR to try and change the supply voltage to the 555 which isn't going to work if that is the case.

Ron

 

[ZOR100]

Hello Ron,

Thankyou, yes the lower circuit on the link page you sent me is exactly the circuit I am using. It allows me to set a final maximum brightness for infrared lighting for a security camera. If it is too bright, night time faces seen in the camera are over exposed (whited out). What I am trying to do is vary the set maximum level with the amount of ambient light rather than a straight switch on at dusk or off at dawn. AsI said, I tried a straight transistor with an LDR to supply the circuits supply but it produced a very low maximum voltage to the LEDS.

I don't know why I cannot add attachments, I click on the paper clip button but my browser cannot find the URL?

Thanks again

 

[ZOR100]

Hello Ron,

I have managed to get it to work. I was using the circuit you had pointed to to drive the LEDS with a 555. I was using a further transistor with an LDR TO provide a variable supply to the 555 circuit dependent on ambient light. However I was making the mistake of using the variable voltage on the output transistor driving the LEDS (collector supply +ve via LEDS) I now have now taken the +ve supply to this transistor from the original 12 volts input, so now much overall voltage swing to the LEDS is almost nothing to 12volts. So I now have control of the final LEDS maximum brightness (via the 555 original bit) and a variance during light to dark ambience.

Hope I have put this in a way you can understand. Sorry for no attachments, thanks for your help.

 

[Reloadron]

I am just glad you have it working.

Trying to envision a described circuit is always somewhat of a challenge. Actually the picture I was seeing was not quite what you were doing. Anyway, happy you got it to come together.

Ron