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Hello everyone, I am relocating a pair of transmission control levers to a different location within a car. At present each lever actuates a pair of SPST microswitches (at the same time) in a voltage/resistor ladder circuit. The diagram is below:

Original circuit The outputs from the circuit are up, down, and a standby line which is used to provide fault tolerance. It is connected to a gearbox ECU and the replacement circuit must match exactly the existing voltage outputs.

There are some constraints I must work with in the new location:

  • There are two wires between the old location and new. A VBATT and ground do exist in the new location;
  • The two levers in the new location each operate a single SPST switch.

I came up with the idea of using the new switches to operate a pair of solid state relays, i.e.:

enter image description here

However this was before I realised there is a voltage drop across the load side of SSRs which means the circuit wouldn't work, because the gearbox ECU would not see the voltages it was expecting.

I am now scratching my head and seeking ideas. Any help appreciated.

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  • \$\begingroup\$ N.B. the 38 ohm reisistors and 4.97v supply are inside the gearbox ECU where the voltage is measured and cannot be changed. They are shown to aid understanding. \$\endgroup\$ – mwstewart Dec 18 '16 at 22:54
  • \$\begingroup\$ What is the bottom end of the top 470 Ohm resistor connected to, and why is it missing in the second diagram? Where are the 470 Ohm resistors located? \$\endgroup\$ – Bruce Abbott Dec 18 '16 at 23:27
  • \$\begingroup\$ Hi Bruce, The implementation is comprised of two PCBs and the manufacturer used the same type for each - to standardise I guess - so the 470 is only connected in the second PCB. I should have removed it from the diagram for clarity but got used to it being there. \$\endgroup\$ – mwstewart Dec 19 '16 at 10:59
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You could use reed relays, which being mechanical would perform just like the existing switches. Make sure they have sufficient lifetime for your needs though.

Alternatively, analog switches can have under 1Ω resistance which should be within your tolerances.

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