How do I control the sensitivity range of an LDR-controlled LED?

Question

I'm trying to make an LED dim according to the ambient light (darker ambient -> brighter LED), through use of an LDR.

I'm using a voltage divider with an NPN transistor (BUL38D). I've tried multiple values for each of the resistors shown, but always seem to end up with a fairly narrow range of light intensity from the LED.

It's either bright or dim, but changes in the LDR (turning the lights off or putting my finger over it) don't seem to change the LED's brightness much, although some change is visible. When I shine my lamp directly at the LDR the LED does turn off.

Is there anything I'm doing wrong? I'm trying to get 20 mA when totally dark (here the LDR is about 400 kΩ) and transistor turned off at LDR = 8 kΩ or so. Do I need a higher voltage?

Edit: I tried the Darlington pair without the rheostat but with a 30 kΩ resistor instead, and after some tweaks it ended up with the sensitivity range I wanted, circuit below. I appreciate all your replies, I learned a lot!

In order to add more LEDs, how should it be done in such a way that they all keep their brightness up?

Answer 1

You can try replacing the 30K resistor with a 500K rheostat with 10K in series, and adding a second transistor to make a Darlington pair.

^{simulate this circuit – Schematic created using CircuitLab}

Answer 2

If you move the LED and the 100 ohm resistor to the emitter path and connect the collector directly to 5V, you remove the voltage gain of the transistor and use only the current gain.

This uses a bigger part of the voltage divider range of 30 kohm and the LDR. You can try other resistor values than 30 kohm to get a better conversion of the LDR resistance.

However the LDR is not a linear sensor in this circuit, solutions with more components (OpAmps) can improve this.

Answer 3

Generally, you want low gain in this circuit; you also want plan what you need the circuit to do.

The LDR conductivity (1/R) is approximately proportional to the incident light. Human perception of 'light' and 'dark' may span many decades -- do you want the LED to work over that range ?

Do you want the total illumination to be constant ? If so, then the sum of LED + incident = constant; the LDR would then have a constant resistance, so your circuit would drive the LED to the point where R(CDS) is constant. If so, then try take Spehro's circuit; remove R3 and R4; place ~ 100 Ω in the emitter of Q2 (may not be necessary). R2 might be better at 100 kΩ. Arrange your hardware so that the LDR is illuminated equally by the LED and ambient light (or use a 2nd LED in series for just that portion).