The 7-segment LED display is too dark when connected to a auto dimming circuit

[Joelam1027]

I'm using the below circuit to build a auto dimming circuit. I was fine when I only use one 7-segment led, but when I connect more 7-segment led (parallel to L1) the light intensity is very low. How to make it brighter and function properly? Thank you.

 

[rjenkinsgb]

You either need a higher-gain transistor or a bigger (lower on-resistance) LDR.
You could also try reducing R2 to 470 Ohms, but I'd not go lower than that.

If you do not get full brightness with the LDR shorted out, the transistor is the problem, it's not got enough gain.

 

[Les Jones]

It looks to me that your circuit will work the opposite way I would expect an auto dimming circuit to work. I would expect the display to be brighter when the LDR had a lower resistance. (Brighter illumination.)

Les.

 

[gophert]

It looks to me that your circuit will work the opposite way I would expect an auto dimming circuit to work. I would expect the display to be brighter when the LDR had a lower resistance. (Brighter illumination.)

Les.

It will decrease resistance from base to ground when the LDR has more light on it and allow more current to flow through the PNP.

To make it brighter, increase the value of R1 to to get a brightness you like.

 

[rjenkinsgb]

To make it brighter, reduce the value of R1 to 22k, 10k etc to get a brightness you like.

Er.. Won't that direct more current away from the base and reduce the output??

 

[Les Jones]

HI gophert,
You are correct and I am wrong. I had just looked at it as a potential divider of the LDR and R2. I had ignored R1 and the connection between the base and the left hand end of R1.
I agree with rjenkingb that R1 needs to be increased to make the LED brighter.

Les.

 

[gophert]

Er.. Won't that direct more current away from the base and reduce the output??

Nope, follow the red line in post #4

 

[Pommie]

Maybe redrawing the circuit might help.

Mike.

 

[rjenkinsgb]

Either way, if it's not providing enough current with the LDR brightly illuminated, the transistor gain is too low.

For a higher current load, possibly a PNP darlington or P channel mosfet would work better, with slight adjustments to the bias resistors for the FET.