Saturday, January 12, 2019

designed dual control schematic

When I realised I needed another dew heater controller (operating under the assumption that the cheapo LED dimmers were not up to the task) I started looking at do-it-yourself solutions.

Found one that I liked using a 556 dual timer integrated circuit (IC) chip. It featured four outputs but offered two independent controls. And the LEDs on the output side would pulse to hint at status or the duty cycle. Those things would take it a couple of notches up from my existing old classic Kendrick Type IV. Oh, and this design could be built with component electronic pieces, many of which I knew I already had. (I'm still not ready to jump into the Arduino pool.)

The author shared his specifications including the input voltage range, 8-18 volts direct-current (recommended 12VDC), and the maximum current for each pair of channels at 3 amperes (1-2A recommended). Good stuff.

custom schematic for dual control dew heater box

Based on the design by Michael Vlasov over at, I built a circuit in ExpressSCH.

[ed: For clarity, so to avoid confusion, this is the final corrected schematic.]

N.B. Express uses a modern style without hops or bumps when wires cross. So, read carefully. If there's not a little square or round bullet, there's no connection.

I ditched the 6 volt output element (as Michael suggested).

I added a fuse to the circuit schematic. He recommended it; I was planning to do so anyway. By the way, the offset of the battery and fuse are meant to show that the power source and fuse are external. The circuit could be powered with a marine battery, booster pack, or power tank attached with a Cigarette Lighter Adapter (CLA). I plan to use a CLA plug with built-in fuse. In other words, the fuse is not in the project box or on the circuit board.

I added reverse-polarity protection with the Schottky diode. Just in case I hook something up wrong one day in the future. (Which will be on the circuit board.)

Aside from those changes, I kept everything the same...

That said, the C4 and C4 electrolytic capacitor values can very. I found a circuit based on a 555 IC that called for 1.5 micro-farad caps. I later tested with 1.0μF and 2.2. It just changes the shape of the duty cycle.

Speaking of the 555 circuit, it showed 10 kilo-ohm resistors on the output of the chip. I tested R5 and R6 with 10KΩ and 2.2K and both worked fine.

The R7 and R8 resistors might be important to change. It will be affected by the current threshold required for the light-emitting diodes (LED) D5 and D6. You might deliberately want to throttle the LEDs so to not be too bright late at night. To that end, you might make a point of using low intensity diodes with diffuse packages.

I considered multi-turn potentiometers for the 1MΩ R1 and R2 components.

Michael's part list showed Q1 and Q2 as 2N2222 transistors. That's a old, obsolete type so you may have to make substitutions with another NPN.

Also TIP31 power transistors are discontinued but you should be able to find equivalents.

I briefly considered changing from the TIP31 type for more power but in the end stayed with it. Also MOSFETs are worth looking at.

I encountered some challenges with the transistor elements in the circuit design tool. When I used the NPN transistor element, I had to assign the pins. Initially, I made an assumption that it didn't matter what the numbers were... Learned a lesson there (which I'll document in another post). To the point here and now: be careful that your pin-outs match your physical component. With transistors, of course, the base, collector, and emitter pins can be anywhere! You'll note by schematic shows different pin layouts for the different purposes.

I also started a board layout file, using ExpressPCB. I knew this would need to be revised once I had all the physical parts, considering form factors, spacing issues, and whether I would arrange parts horizontally or vertically on the perforated board [ed: which generated a significant error that would haunt me].

One final note: your circuit can be tested without heaters attached...


Next: gathered the parts.

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