This is just a modification of Dave's Dew Zapper circuit. It dispenses with the floating ground but is functionally equivalent; but it does provide two seperate heat outputs, each one controllable seperately. This is done using an LM556 dual-timer in place of the LM555 timer used in the Dew Zapper.
The prototype board
This is the board showing all of the 'external' connections. The two 50K pots are connected oppositely to each other. That is, looking at the top of the board with the connections for the pots at the top the one on the left is connected reversed to the one on the right. The power outputs (black leads) go to your choice of connector, or connectors in parallel, then to ground. The thick red lead brings 12v power to the two BUZ71A powerFETs. The power for the chip and circuitry is brought in seperately on the prototypes shown via the two-wire lead running from the 'bottom' of the board. This is dispensed on the layout at the bottom where only a single +12v lead goes to the board and a single lead to ground for the timer circuitry. The heaters still go to their own grounds.
This is a closer look at the board showing the layout of the components. On the final board there are three added 2K2 resistors for the LEDs, a couple of diodes to keep the heat power part going the right direction and a capacitor/diode to protect the timer section.
Here's the solder side of the prototype board. Note that there are no mounting holes in it; I'll add them when I figure out how I'm going to mount the thing in a box.
Final layout
Note that the layout calls for two IRF530A FETs. My supplier didn't have any so I used BUZ71As. They're equivalent. The board shown in the pictures is the 'test of function' prototype. The layout above has been cleaned up, some protection added and places to connect two 'power level' LEDs and a 'power on' LED.
Here is a link to the PhotoShop image you can use to print out the solder-side of the board for toner-transfer board creation. Print it at 161 dpi.
Schematic
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