Dimming one light while brightening another

Thread Starter


Joined Nov 14, 2017
I want to simulate the suns rays striking the surface of the moon for an astronomy club. My idea is to use two lamps either led or bulbs, (led preferred) placed 90° apart or 45°either side of the vertical where a camera will point down on a crater surface. Turning a rotary switch will dim one lamp while brightening the other so that at the mid position both lamps are equally lit. This will show the change in shadow detail when viewed from above.
Could anyone please assist me in the circuit required as I just can't work this out?
Many thanks


Joined Oct 29, 2013
Lots of possibilities.. Use a small board like an Arduino, and a potentiometer (knob) connected to one of the analog inputs, as well as a pull down resistor. Use 2 MOSFETs, one to control each light. Connect the gate from each MOSFET to an output on the arduino, and put a pulldown resistor on each gate as well. Now use PWM to the MOSFETs to control the brightness of the lights. As the analog input pin to the arduino reads more voltage (you turn the knob up) increase the on-time for one MOSFET and decrease it for the other.

If you want to do it as an all analog circuit (no microcontroller), then maybe you could use a couple of n-channel MOSFETs for the lights, with the gates connected to your pot, and pull-down resistors connected to each MOSFET gate. Use an inverter on one of the MOSFET gates. Now as you turn the knob up, one will get brighter and the other dimmer. You'll have to play with the resistor values. Also you might need heatsinks on the MOSFETs in this configuration, they'll get hot.

I'm assuming you're using DC power for the lights. Are you using AC?


Joined Oct 2, 2009
There are a number of ways to do this. You can do this electronically using analog or digital circuits.

You can also do this with a light dimmer switch.

In the days of 35mm slide projectors, I gave slide shows with two projectors. Instead of having the projectors turn on and off abruptly, it was more pleasant to the viewer to have one projector fade out while the other faded in. This was accomplished using two ganged pots, i.e. two rotary pots mounted on the same shaft.


Joined Sep 9, 2010
I'd look at using a PWM (pulse width modulation) controller and two MOSFET switches, one N and one P, driven by the PWM output. One LED would be controlled by the P MOSFET, so it would be on when the signal goes low. The other LED would be controlled by the N MOSFET and would turn on when the PWM signal goes high. This way, the duty cycle seen by one LED is complementary to the duty cycle seen by the other, for instance 10% and 90%, or 60% and 40%.

You could avoid building the PWM controller by just buying a cheap one on eBay and adding the transistors and LEDs yourself. If you use 12V for the supply, you could put 3 LEDs (and a current-limiting resistor) in series on each leg.


Joined May 15, 2009
I would use a couple of Darlington transistors (discrete or integrated) connected as current sources (that is with a largish emitter resistor) and simply feed them from each side of a linear pot with the wiper going to ground. The bases also need a bias resistor to your supply line to turn them on. That way gives you very simple control including, when the wiper gets to ground one end of the pot or the other, complete cut-out of the particular LED compared to simple PWM's that might not give you 0-100% control. For example: three 3V LEDs in series operate from 12V with a spare 3V drop. A Darlington would drop 1V approx. leaving 2V to set the current. If you have 350mA LEDs that needs 5.6ohm resistors,but if you use smaller LEDs use higher resistors and if you use signal LEDs (20mA) maybe only one transistor is needed, not a Darlington. The pot should carry about 10mA as the base current is low,(this being about 10x the base current, so use a higher pot for lower current LEDs) so that needs to be about 300 ohms per side or 1k in total should work, with a pull-up resistor of 1.5k on each end of the pot too (again, higher for lower currents). In theory this could drive the "on" LED at twice the normal current except it won't if the voltage is limited to 12V/9V from the battery and LEDs, but you could double the resistor to check it won't blow up.
Many times I have seen "LEDS need PWM" but although there are some slight colour changes, good LEDs will work with analog dimming. And the benefit is that there is no interference to suppress.But you may need small heatsinks for the transistors.