Electronic Pressure switch

        I'm using a DIY electronic adjustable pressure switch for the onboard air system in my car. The previous design worked well and has been in use since 2008 but I'm going to upgrade to a solid state sender in the future so a new one was designed that can use both the older mechanical and newer electronic pressure senders. I designed a new circuit and it is shown below.

        The sender used is a typical oil pressure sender good for about 150psi. At 0psi, the terminals read 0 ohms. At around 120psi, it reads 133ohms. Since it is variable resistance, I decided to use a current source to provide constant 22mA current across the sender and then read the resulting voltage. A regulated 5V output is also added to use for the newer 3-wire solid state senders.

        The input signal goes to an input conditioning stage consisting of a voltage swing limiter (R2 and D3), and a low pass filter (R3 and C3) to slow down the response time before going to an op amp buffer/ variable gain stage. The gain stage is added incase the sender used has a low output voltage. By using a jumper placed in JP1, the op amp can be set to unity gain (short pins 2 & 3) or variable gain (short pins 1 & 2; gain = 1.42 to 11). Gain is adjusted via R5.

        A test point (Pad1) is added to aid in setting the gain for maximum op amp voltage swing at the maximum pressure to be used at. If using variable gain, once the transducer is at the maximum working pressure, adjust gain so that the voltage at the test point is about 3-3.5V.

        The second op amp is used as a comparator. The 2.5V threshold voltage is derived from the R8 and R9 voltage divider with R14 providing about +/-56mV hysteresis. Pressure set point is adjusted via R7.

        The controller threshold can be adjusted from 0.227V - 0.322V (max gain), 1.76V - 2.49V (min gain) and 2.5V to 3.54V (unity gain) so there is a wide overall threshold voltage range of 0.227V to 3.54V that can be accomodated. If the sender output is higher than the 3.54V maximum, adding an 8.2K resistor from the anode of D3 to ground can be added to act as a voltage divider and changes the threshold voltage range to 0.454V - 7.08V.

        D1 and D2 doesn't necessarily have to be a schottky. I just had a bunch of those but the lowly 1N4001 diode can be used. For the P channel FET, any P channel FET with adequate rating should be OK. IRF9540 would work fine in the circuit.

        The prototype is built using SMD components because it is more convenient that there isn't a lot of holes needed to be drilled on the PCB.

Completed module:

Bottom side:

1/2 case of a laptop brick serves as a quick and dirty enclosure:

Ready to be installed in the car. Since it will be inside the kickpanel, being open is not a problem:

Testing and calibrating the pressure switch. orange and black wires are power, yellow and grey on the left are going to the relay in the engine bay, twisted yellow and grey go to the sender under the dash.

The sender under the dash. There is a 4mm tube from the tank at the back going to the left of the manifold holding the sender. The tube on the right goes up to the dash where the gauge is:

Controller powered up:

Got some OEM switches so that they would look stock. (from left to right: stock/truck horn/train horn switch; compressor switch; footwell lights switch):

Controller module tucked under the sender:

The value of the hysteresis resistor gives about 15psi differential from on and off so the switch turns on at 110psi and off at 125psi.

With the kickpanel installed, the module is hidden and protected. Nothing is visible when looking from the driver seat and the pedals are unobstructed.:

       20 Jan 2013:

The solid state pressure transducer has arrived. But the mechanical sender still works OK, and I'm too lazy at the moment so testing will be done some other time.

Page created and copyright R.Quan © 10 June 2011.