Digital Rheostat

[ngauger]

I hope this is the right forum. I am trying to find or define a digitally controlled rheostat or voltage controller. Ideally there would be a four to eight bit digital input and the voltage output would be proportional across standard house voltage 0 to 120 v. I want to use it for a light intensity or dimmer circuit. I have been unable to find such a commercial product. I have attached a generic circuit I found but I lack the knowledge to pick an appropriate opamp or determine the various resistor values. Hopefully, this interests someone who can help.

[kxenos]

Welcome to the forum. This circuit is for DC. For 120VAC you need a dimmer with triac. A digital dimmer is a rather difficult circuit for someone to start exploring electronics.

[ngauger]

kxenos
Thanks for the input. I did not pick this design as a random place to start with electronics. I am a model railroader and am looking for something to meet a function I desire for the hobby. Do you have some suggestions of where to look for existing designs or even commercial manufacturers. Of couse, I am looking for something relatively cheap. Thanks again.

[Seekonk]

I would seriously think about using a UNO if you need multiple control.  I get these microcontrollers shipped from China on ebay for about $8 and they are pretty easy to use.  For AC lighting you want to purchase AC solid state relays that are NOT zero voltage switched, that means you can trigger them on at any time.  That will allow you to make a phase angle control for light dimming.     If you have low voltage DC lamps, simply drive a FET and use one of the UNO PWM pins.

[Paul Moir]

Take a standard light dimmer, replace the pot with a CdS cell and put it back together.  Now you have a light controlled light dimmer.  Couple the CdS cell with an LED and feed the LED from a DAC or even a 2R2 resistor ladder.  The ladder might make more sense because you could play with resistor values to get the right light output per step.   You may want to drive the ladder with something like a 74HC244 or if you want latching an 74HC573.
Alternatively you could PWM the LED if that's easier than the DAC.