In my previous article we learned about these wonderful ICs 2N2907/2N2917 which are basically frequency to voltage converters and therefore perfectly become suitable for all frequency measuring related applications.
Referring to the simple speedometer circuit diagram below, we can see the IC LM2907 occupying the center stage in the configuration and forms the main detector device.
As can be see the internal of the IC comprises an input differential comparator, followed by a charge pump, an opamp buffer and a emitter follower amplifier stage.
Pin#1 of the IC becomes the tachometer input and accepts the magnetic pick up information in the form of varying magnetic electrical pulses.
The differential opamp compares the input with its ground referenced inverting input and amplifies even the minutest pulse detected at pin#1.
The function of the charge pump stage is to hold and pump the above amplified signals such that with increasing frequency the corresponding amplified output from the differential stage is sustained and boosted proportionately across the next opamp buffer stage.
This is achieved by the presence of the capacitors at pin#2 and the resistor at pin#3 of the IC.
The final proportional boosted potential is fed through the emitter follower transistor across the 10k resistor.
As given in the diagram, the output sensitivity here is around 1V for every 67Hz, meaning if the frequency reaches 67+67 = 134, would result a linear output of 2V and so forth.
To be more precise, the output voltage can be simply calculated using the formula:
This can be directly read by connecting an analogue moving coil voltmeter across the 10k resistor.
The magnetic pick-up for the proposed speedometer circuit could be made by winding 50 turns of 30SWG wire over a small ferrite ring. The wheel should be complemented with an appropriately dimensioned magnet stuck or fixed over the rim such that the two elements come face to face once on every single rotation.