How the Tach Amp works

[OldPokey]

First of all here are two images of the schematic I drew:

Small: Small
Large: BIG!

What the Tach Amp does is to take in the signal from the sensor that detects the pin on the crank pulley and produces a 3 mS pulse to the tachometer every time the ping goes past the sensor.

How it does it:

There are three parts to the circuit.
1. Voltage Conditioning
2. Sensor interface.
3. Amplifier

Voltage Conditioning

The 14V nominal voltage is first run through a 32 Ohm resistor to limit the maximum current the amplifier will draw. That voltage is filtered by a 100 microfarad capacitor and fed to a 360 Ohm resistor and then to the cathode of a 5.6 V zener diode. The 360 Ohm resistor and zener diode maintain a set voltage at the point where the two come together. This regulated voltage is fed to the LM1815.

Sensor Interface

The sensor is called a Variable Reluctance Sensor, which is nothing more than a small electromagnet. When a chunk of steel (the pin on the crank pulley for instance) passes through this magnetic field, the voltage across the sensor changes slightly. The 169 Ohm resistor attached to the Emitter of the transistor sets the maximum current that can flow through the transistor and then to the sensor. The Transistor itself and the 830 Ohm resistor from its base lead to ground set up a constant current source to reduce noise from the sensor (if the current flowing through the sensor isn't constant, you would end up with lots of noise and voltage fluctuations into the Amplifier). The 2.2 microfarad capacitor blocks the DC voltage from the input to the amplifier, but allows the pulses from the sensor to get through.

To condition the signal before it enters the amplifier, it is low-pass filtered by the circuit made up of the two 18K resistors and the 0.0033 microfarad capacitor. This filter cuts out any high frequency noise that may cause the tachometer to produce bogus readings.

Amplifier.

The LM1815 Amplifier takes the signal, amplifies it and compares it to a threshold it sets based on the average level of the incoming signal. The threshold is maintained by the 4.7 microfarad capacitor attached to pin 7. The 1.2 M resistor slowly bleeds off the charge from the capacitor, without it the threshold would just keep climbing - if it were too small a value the threshold would never get off the ground and the signal to the tachometer would be too noisy. The 4.7K resistor between pins 5 and 8 sets the initial threshold. When the voltage rises above the threshold, the LM1815 arms itself so that when the signal falls back below the threshold it can trigger an output pulse.

To make sure the signal to the tachometer is of a set pulse width, the LM1815 incorporates a timer that is set to produce a 3 millisecond pulse via the time constant set up by the Resistor/Capacitor pair made up of the 470K resistor and 0.01 microfarad capacitor attached to pin 14.

Since the output from 12 can only pull to ground, the 27K resistor from pin 12 to 8 pulls the output pin high unless the LM1815 is actively pulling it low.

Pretty good for something that fits into less than 1.5 inches square!