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# Tachometer circuit using LM2907 LM2917 frequency to voltage converter

Using LM2907 ,LM2917 monolithic frequency to voltage converter can be designed a very simple tachometer circuit , that can be used in various electronic projects .

Both input and output interface circuitry is included on chip so that a minimum number of additional components is required to complete the function. In keeping with the systems building block concept, these devices provide an output voltage which is proportional to input frequency and provide zero output at zero frequency. In addition, the input may be referred to ground. The devices are designed to operate from a single supply voltage, which makes them particularly suitable for battery operation.

This tachometer circuit based on the LM2907 IC can be used to provide zero crossing datum to a digital system. At each zero crossing of the input signal the charge pump changes the state of capacitor C1 and provides a one-shot pulse into the zener diode at pin 3.
The width of this pulse is controlled by the internal current of pin 2 and the size of capacitor C1 as well as by the supply voltage.

Since a pulse is generated by each zero crossing of the input signal we call this a ``two-shot'' instead of a ``one-shot'' device and this can be used for doubling the frequency that is presented to the microprocessor control system.
This electronic tachometer circuit project can be powered from a 12 , 15 volt DC power supply circuit .
Input can be from plus /minus 20 mV to plus/minus 28V . Pulse width is equal with (VCC/2)x(C1/I2) and the Pulse height is equal with VZENER .

### Sorry I am a little

Sorry I am a little confused.... first you say: quote [...capacitor C1 and provides a one-shot pulse into the zener]- Then you say quote "[each zero crossing of the input signal we call this a ``two-shot'' instead of a ``one-shot'' device]". Why this discrepancy and if it is the latter, why would it be called a "two-shot" if the pulse is only generated on each crossing...are there two crossings per revolution? Thanks Joan