Handbrake bypass switch

I bought a Pioneer radio and wanted to bypass the brake detection circuit to watch DVD’s. The unit has a wire that needs to detect nothing on the wire when you turn the vehicle on, and then a connection to Ground (handbrake on) before enabling the DVD player.

On the internet you can find a circuit where they use a relay to bypass it. The circuit looks like this:

Relay handbrake bypass

 

It works as follows:

  1. When you turn on the vehicle, there is no power on the Blue (Amp/delayed power) wire, so the relay is not energised and the Green (handbrake detect) wire detects nothing (connected to the Normally Open pin of relay)
  2. A short while later, power is delivered to the blue wire, energising the relay (switching it on)  and now the Green wire detects ground.
  3. The Blue wire is now permanently powered (that is, until you switch the vehicle off), so the Green wire keeps on detecting Ground (handbrake connected)

This circuit works, but for me the relay solution looked too clumsy and wasted unnecessary power keeping the coil energised, so I made the same circuit with a transistor.

Transistor brake bypass

This circuit works basically the same:

  1. When you turn on the vehicle, there is no power on the Blue (Amp/delayed power) wire, so the transistor is turned off and the Green (handbrake detect) wire detects nothing
  2.  A short while later, power is delivered to the blue wire, switching the transistor on,  and now the Green wire is connected to  ground.
  3. The Blue wire is now permanently powered, so the Green wire stays connected to ground (handbrake connected)

The transistor circuit is much smaller than the relay circuit and consumes  almost no current!