Once the board has been etched, drilled and tin plated, if you decide to do this, we need to complete the via connections. The PCB was designed so that it can be made at home without using through plating. Wire links need to be soldered between the top and bottom plane wherever a via is present on the artwork. The method I use is to use the legs from 1/4 watt resistors as the links. I cut 4 sections of resistor legs and place one under each corner of the PCB. This will lift the board off your work surface by the thickness of the wire. The linking wire is then pushed through the via hole and trimmed as close to the surface of the board, but still long enough that we can solder it in place. Once all the links have been soldered on the once side we flip the board over and solder the second side. All the links should already be of the correct length on this side since we had a space between the board and work surface.

The next step is to populate all the surface mount capacitors and the two SOT23 package voltage reference diodes. The surface mount IC’s are next followed by the resistors. All the resistors and the one diode need to be soldered on both the top and bottom layers of the board, certain tracks are connected between the two planes using the resistor legs.

Next to be soldered are the two fuses, I use two pins from a Tulip dip socket that have been cut out so that only the metal pin is left. These also need to be soldered on both the top and bottom. You could forego the socket pin, but it makes changing the fuse easier should it ever fail.

All the TO-220 packages are next in line. They also need to be soldered on both planes of the PCB.

The second to last parts to be connected are the electrolytic capacitors, the 40 pin IC socket and all the connectors. These all only need soldering on the bottom of the board.

Finally we can connect the Humidity sensor. Be very careful with this part. Any fingerprints or any other contaminant will quickly cause it to fail, I learnt the hard way after spending hours trying to track down why my unit was acting up. I recomend getting the type with a built in dust cover.

The small heatsink can now be installed on the 7805 voltage regulator.

Quick Tests

Before inserting the PIC we need to check three voltages.

Hook up the input 12V to the power connector and check for 5V between pins 11 (positive) and 12 (negative) of the 40 pin IC socket. Anything close to 5V is correct. If we don’t have 5V then you need to check that the input protection diode was inserted correctly or that there aren’t any shorts on the PCB somewhere.

Next we will confirm that the two reference voltages are correct. We should measure about 2V between pins 4 (positive) and 12 (negative) of the 40 pin IC socket. We should also get 4V between pins 5 (positive) and 12 (negative) of the 40 pin IC socket.

Once these tests have passed we can insert the microncontroller into the IC socket, if you are going to use an external programmmer and not the ICSP connector you need to program the PIC first though.


PCB Wiring


Temperature Sensors P4

Any connector that has at least 8 pins can be used for connecting the 4 sensors to the unit, eg DB9 chassis mount etc.

Connect the wires from the P4 connector on the PCB in the same sequence onto the chassis connector. An external cable can then used used to connect to each individual sensor.

Temperature sensor number 4 is the ambient sensor, number three is for the main mirror and fan combination. Numbers one and two are for heater channels one and two.

Serial Connector P5

A DB9 female connector is wired up to the P5 connector using the following pinouts.

P5                  DB9 Female

2                     2

3                     3

4                     5

Fans P3

You can use any style connector for this, I just used two  RCA style sockets.

Pins 3 and 4 of P3 are connected to the positive input of the fans, with pins 1 and 2 going to the negative connection.


Aux Relays P2

Again any style connector can be used.

Pins 4 and 2 will be connected to the external voltage used to control the relays. Pins 1 and 3 will connect to the positive input of the relay. The relays negative pin must be connected to ground. Only DC relays can be used.


Heaters P1

Again any style connector can be used. Normal practice is to use a RCA socket. This is what I have done, even though they are about the worst connector to use. Since I switch the ground line on and off we have to make sure that if the RCA sockets are mounted on a metallic plate that we isolate them from each other somehow. I made up some plastic bushes that fit into the hole drilled to hold the sockets. If using a non conductive back plate then it will work fine. It’s important to make sure that there is no electrical continuity between the external contact of the various RCA sockets. The same is true for the fans if using the same connectors. If possible try and pick some isolated style connector. If they are electrically connected together all the heaters will switch on together when any one heater is told to switch on.

Pins 5 to 8 of P1 is the 12V which needs to go to each heater. Pins 1 to 4 are the switched ground lines that will connect to the other side of the heater.

Power P6

I just brought the external power via an on/off switch monuted on the casing through a chassis mounted fuse holder (about 8A is fine) onto the positive and negative terminals of this connector.


Pins 1 to 3 connect to each LED’s cathode. The anode is connected to pins 4 to 6.


External Cables

Serial Cable Pinouts

The serial cable is used to update the controllers firmware from a PC, without having to use a ICSP or removing the PIC and using an external programmer.

DB9 Female                DB9 Male

2                                    3

3                                    2

5                                    5


Temperature Sensors

Each sensor will require two wires that are then connected to your sensors chassis connector.

Follow the connections as shown in the next image. Use heat shrink to isolate the 3 pins once the two wires have been soldered on.


Controller Box

I’ll leave it up to you how you’ll mount the PCB and connectors etc. Since the humidity sensor is mounted inside an enclosed box I drilled a pattern of holes on the top and bottom surfaces of the box. This is to allow outside air to pass over the humidity sensor to get an accurate reading. I also mounted a fan over one of the section of holes to suck air out and draw air in on the other side. Not sure if this is necessary though. You can see my setup in the next few screen shots.

A shot of the finished wiring, a bit unclear but shows the general idea.


A picture of the rear showing the connectors I used. The fuse holder is to the far left of the rear panel. The ventilation holes are visible on the top. I have mounted a fan underneath these that is orientated to blow air out of the top. An identical set of holes are drilled into the bottom of the box. Air is drawn in through these.


A view of the front of the box showing the three LED’s the on/off switch and the USB connector.

The LED’s show the following.

  • One to indicate a flat battery
  • One to indicate that one of the heaters has drawn too much current
  • The last one flashes to indicate the chance of dew. The faster it flashes the higher the chance that dew will form.




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