Tag Archives: electricity


Jeep Seat Heaters

Once you own a car with seat heaters, it’s hard to go back. The old VW had heated seats; the new Jeep did not. Clearly, this state of affairs could not stand.

I found that I could get some nice, neoprene seat covers with built-in seat heaters made by Wet Okole. The Wet Okoles came with heating elements in both the butt-area and the back-area, whereas some aftermarket heaters only heat the butt. I used Quadratec’s “designer” for Wet Okoles. When the seat covers arrived, I installed them, and there was much rejoicing.

Wet Okole Seat Covers

Each seat had a cigarette lighter plug for power, and a push-button switch that allowed setting the heater element to Off, Low, Medium, or High.

Original Switch

Here we get to the problem: While convenient for a quick connection, I didn’t want wires dangling from the seats to the cigarette lighter. Further, I only had one cigarette lighter. Even splitting the circuit for the cigarette lighter wasn’t a great solution, as each seat could potentially draw a little more than 10 amps, and the lighter was on a 20 amp fuse. Splitting the circuit would mean that I’d risk blowing the fuse each time both seats were on full. The Jeep conveniently has a spare 20 amp circuit on the fuse block behind the dash, but it’s an unswitched circuit. If I used that, it’d only be a matter of time before I’d leave the car with a seat heater on, and I’d come back to a dead battery.

After using the seat heaters for a while, I decided that they were keepers, so it was worth investing in a more permanent solution to the power problem — and a solution that would allow both seats to be heated at the same time. I needed to add at least two new 15 or 20 amp, switched circuits to the car, and I wanted controls that were integrated into the dashboard somehow.

For the controls, I started looking around for switches that could put into some existing blanks on the dash. However, after bashing in a heater vent (by transporting some furniture in the passenger seat), I stumbled upon the perfect solution in a Daystar replacement vent with an integrated switch panel.

Vent Switches

The drawback of the rocker switches is that I would no longer have Low or Medium settings. The seat heaters would either be off, or fully on. But really, who need a “lightly warmed” bum in winter? If it’s cold enough to turn ’em on, turn ’em on ALL THE WAY, I say!

For the circuits, I found that Painless Performance makes three- and seven-circuit add-on fuse blocks. I decided to go with the seven-circuit block to give me room for expansion in future, yet-to-be-conceived projects (such as my Arduino-based trip computer). That gave me four new switched circuits, and three new constant circuits, all at 20 amps.

The parts arrived, and so I got to connecting all the pieces. The trickiest decision was where to mount the new fuse block. I had initially intended to mount it behind the glove compartment, next to the existing internal fuse block, but there wasn’t enough room. Perhaps I could have found another spot behind the dash, but I didn’t want to put it somewhere that would require ripping open the dash to access it (in case I should need to replace a fuse). There was an empty spot in the engine compartment (for a second battery, I suppose, to power a winch that I’m unlikely to add) that seemed like a good candidate. Being in the engine compartment, I wanted to add a bit of protection to the block, as it wasn’t marketed as a weatherproof component. So rather than mounting it directly, I mounted it to the inside of a small tupperware bin. I drilled a few ventilation holes in the bin, and a larger hole to run the wires, then mounted the bin in the engine compartment.

Placement of fuse block

The new fuse block is in a ventilated tupperware bin, mounted near the back-driver-side of the engine compartment.

The fuse block had three sets of wires:

  1. Two wires to connect to the positive and negative poles of the battery to power the circuits.
  2. A single wire that needed to be connected to an existing switched circuit. This wire poweres an internal relay that controlled the switching of the four switched circuits in the fuse block.
  3. Seven hot wires for the seven new circuits.

Since the fuse block was already in the engine compartment, running the first set of wires to the battery was fairly trivial. The rest of the wires, though, had to make it into the cabin, which meant getting them through the firewall. I spent more than a few minutes looking for an accessible, existing run through the firewall, and was met with no success. I refered to Dr. Google, and learned that a hard, rubber plug near the gas pedal is the preferred channel — just make a hole straight through it. I made the hole, and pulled the wires through.

Wires run through the firewall

To get the wires through the firewall, I had to make a hole in a rubber plug that seemed to exist for exactly that purpose.

For the relay wire, I tapped into the hot line for the cigarette lighter — I just cut away a centimeter of insulation, joined the relay wire, and wrapped it up neat and tidy with electrical tape. The remainder of the work consisted of running wires up and down behind the dash: hot circuit wires to switches, switches to positive wires for the seats, switches to ground, seats to ground.

As of this blog post, the seats have been keeping my bum warm for more than two winters. A boy could hardly ask for more.



Blinking LED Circuit

As part of a larger project, I needed a circuit to blink an LED. It’s a simple task, and there are plenty of existing designs. But having almost no experience with circuit design, I wanted to make my own. Further, I wanted to make it with basic components — no ICs. An integrated circuit, like a 555, would take the fun out of it!

As is the case with most oscillators of this sort, the charge/discharge cycle of a capacitor acts as a switch to turn the LED on and off. I sketched out a few ideas, and eventually arrived at this:

Blinking LED Circuit

The blinking happens because the circuit oscillates between several states:

  1. Current goes through PNP transistor Q2 to the anode side of capacitor C1. As C1 charges, the cathode side drains through resistor R1 to ground. In doing so, it also puts “pressure” on the base of PNP transistor Q1, preventing current from flowing through it.
  2. After C1 is fully charged, its cathode side will no longer be draining, so there will be nothing preventing flow from the base of Q1 through R1 to ground. At that point, Q1 will start to conduct to the big loop, which will cause three things to happen.
    1. Current will flow to the base of NPN transistor Q3, which will allow C1 to slowly discharge through resistor R3. Also, once Q3 is conducting, the drain on the base of Q1 will increase as current flows to the cathode side of C1.
    2. Current will flow through diode D1, to put “pressure” on the base of Q2, thereby preventing further charging of C1 from the voltage source.
    3. Current will flow through light emitting diode D2, causing it to light up.
  3. Once C1 is fully drained, the base of Q1 will only drain through R1. If R1 has a high enough value, the output of the collector of Q1 will fall below the threshold to block the base of Q2. When that happens, current will once again flow through Q2, C1 will start charging again, Q1 will stop conducting, the LED will turn off, and we return to step 1.

So that was my theory. My next step was to build it, and figure out the right values for all the components. I breadboarded it like this:

Blinking LED Circuit

Voltage: +5
Q1: (PNP) 9015
Q2: (PNP) 9015
Q3: (NPN) PN2222
R1: 10KΩ
R2: 10KΩ
R3: 680Ω
R4: 68Ω
C1: 470µF
D1: 1N4148
D2: Blue, 3.7V, 20mA

That resulted in a flash rate of 1.3Hz. The blinking speed can be adjusted by changing the capacitor and/or changing the values of R1 and R2. I swapped C1 for a 22µF cap, and the flash rate increased dramatically, perhaps to something between 20 and 30Hz. So I swapped R1 and R2 for 100KΩ, and the flash rate returned to something around 1.6Hz. Ideally, the capacitor should be very small, since it is essentially charging, then dumping its charge in every cycle. I’d like to try to use a much smaller capacitor (with larger R1 and R2 values) to see if I can maintain enough current in that part of the circuit to control the functioning of the transistors. I used a blue, 3.7V LED on a 5V circuit. The LED can be changed, as long as R4 is changed as well to ensure the proper current for the LED’s voltage.

It’s not the simplest circuit of this sort, and I’m sure that I’ve screwed up at least part of the analysis. But as someone who only knows as much about circuit design as he could find on the internet, and a couple of books (namely, Getting Started in Electronics by Forrest Mims, and Starting Electronics by Keith Brindley), I was fairly pleased with myself for making this work.