I'm connecting to an arduino or raspberry pi and would like to have 256 io pins. I found a 16bit io expander which has 8 unique addresses for 128 but that's the max I can do on i2c. Any other thoughts on how to expand my io ports?
Electronic – arduino – Best way to get 256 io pins
arduinoioraspberry pi
Related Solutions
Here is a circuit that should work:
Your basic idea of using a high side PNP switch was fine. The problem with it is that this exposes the high voltage to the micro pin.
In this circuit, Q2 is the high side switch that turns the 12V output on or off. Q1 switches the high side switch, thereby isolating the micro from the 12V on the base of Q2.
When the base of Q1 is held at 5V, the emitter will be 4.3V, so there will be 1mA thru R1. Most of this also flows thru the collector of Q1, which thereby acts like a switchable 1mA current sink. Most of this can only come from the base of Q2. Figuring the two transistors each have a gain of at least 50, then the output is good for at least 45mA for Q2 to stay in saturation. The purpose of R3 is only to make sure Q2 is really off when Q1 is off.
It seems that the sensor is highly non-linear and this is good in our case.
You can try to connect these sensors to the digital inputs of the RaPi, if they have some hysteresis (AFAIK, the GPIO inputs have a hysteresis). The schematic if the following:
simulate this circuit – Schematic created using CircuitLab
The value of R1 is the same as the resistance of the sensor on the threshold point. Determine it experimentally.
The capacitor C1 is to reduce the induced EMI. One approximate value is 100nF but it may vary.
Mount the resistors and capacitors as close to the CPU board as possible. Use shielded cable (low frequency) to connect the sensors. The shield must be the ground wire.
If the EMI are too big, some software processing of the false positives can be made.
If RaPi has no enough inputs (80) you should make some multiplexing. Note, that in this case, some buffers with hysteresis have to be used - 74HC7540 is good choice. You can use 10 of them and connect all outputs together and control the 3-rd state inputs by GPIO outputs. This way with 10 outputs and 8 inputs you can control 80 pressure sensors.
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Best Answer
You can put an I2C Multiplexer in there between the Arduino and your devices, then only put one of each address on each downstream bus. A 1-to-2 Multiplexer that I've used with Arduino before is PCA9540BD,118. That will get you to 256 with the hardware and interfaces you are already using with the addition of one more component.