I needed a low cost 3/4 channel dew controller and after going through various diy circuits I built this simple picaxe microcontroller based dew controller that uses just one mcu but can control multiple heaters independently.
Thanks to Fred (Bassnut) for his simple 08m circuit and code that I based this on.
Total cost should be under $40 for 4 channel.
Rather than pwm, I went with simple on/off method as above, so frequency is around 2 to 4 Hz at the most I'm not sure if that can generate rfi.
Advantage of an mcu over a 555 circuit or pwm motor speed controller is fewer components for the same output.
since the mcu is a single core, it isn't parallel tasking to control each channel independently (which the m2 variants are pseudo capable of), but uses the simple method of keeping all channels synced for their "off" states.
Each channel has its own pot and FET and can have different "on" periods during the time taken for one loop, thus independent amount of heat.
I've tried to depict the pulse width below.
"t" is the duration of the loop at the end of which all outputs go low and the sequence starts again. So each channel can remain "on" for different periods but go off simultaneously.
The "on" time is based on the ADC value of individual potentiometers, and the mcu output switches an FET to which the heaters are connected.
|----|___|----|___|----|___| Heater A
|--|_____|--|_____|--|_____| Heater B
I will upload a circuit shortly, but parts list for the two channel controller are
1)Picaxe 14M (recommend using 20x2/20m2 as they have additional ADC inputs)
2)7805 Regulator - Same 12V heater supply is used for the picaxe (as its 5v)
3)0.1mfd and 22mfd cap
4)10k Pot x 2
5)N channel FET x 2
6_Heatsink x 2
7)10k resistors x 3
8)22k x 1
9)Stereo Plug and socket (for programming)
LED's are optional, but I added them at the output of the FET as visual indicators which is very handy
LED x 2
4.7k resistor x 2
General purpose PCB
To add more channels, just use a 20m2 chip (costs $3 more than 14m2 or 18m2) with as many pots as ADC inputs. You could add on additional FET's on each channel.
FET's I used are "N" channel at this stage, but I will change it to "P" Channel to reduce risks as raised by wasyoungonce(Brendan)