Introduction: Differential Sensor Biasing
This instructable shows how you can make a differential sensor biasing circuit.
Differential biasing allows the power supply and EMI noise cancellations for the two inputs.
This circuit is obsolete. There are matched resistor IC bridges sold on the internet nowadays.
A single biasing bridge is made from four resistors. I used one potentiometer to replace two resistors at the bottom of the circuit.
Unfortunately, this circuit only allows matching of voltages and only partial matching of resistance for the two channels. The ideal circuit would consist of four potentiometers.
You can make circuit 1 or circuit 2.
The sensor can be microphone, light-dependent resistor (LDR), a photodiode or phototransistor.
components: cardboard, wires.
Circuit 1: 20 kohm potentionmeter, 10 kohm resistor - 2.
Circuit 2: 10 kohm potentionmeter, 4.7 kohm resistor - 2.
tools: wire stripper, scissors, two-channel power supply (you can use voltage regulators or a few batteries).
optional parts: matrix board, solder.
optional tools: soldering iron, low power dual-channel power supply, hole puncher.
Step 1: Design the Circuit
The circuit needs to be connected to the power supply.
You can Thevenin's Equivalent analysis for this circuit.
You can now see that matching resistances is impossible. You can reduce the cost of this circuit by using a single power supply and connecting the two power nodes together. However, this will defeat the whole purpose of differential biasing where the two channel resistances are equal or at least partially matched.
Step 2: Make the Circuit
Because the circuit is so simple you do not need a matrix board. However, it could be a bit stressful if the cardboard is very thick like the one that I used.
You can punch holes with scissors.
I used an old 20 kohm variable resistor from an old electronic appliance.
Step 3: Testing the Circuit
Step 1: Disconnect the two power supply inputs, connect the two yellow wires together and connect to the first power supply channel.
Step 2: Short the inputs to prevent the influence of EMI noise.
Matching Thevenin's voltages
Step 3: Vary the potentiometer until the voltage of both outputs is equal.
Step 4: Vary the power supply voltage to bias the sensor.
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