This is a take off from the previously posted bicycle anti-theft alarm. One commenter indicated a desire to adapt it to a threshold step plate vibration detector to count patrons entering a place of business. The circuit is now adapted for that application. Note that there are a myriad of other applications for this circuit.
It consists of a piezoelectric film transducer, charge amplifier, charge pump detector, Schmitt trigger and counter driver.
Impulse Counter Schematic
Bill of material
Surplus counter on eBay
eBay is perhaps the best source of inexpensive counters. The best ones have a reset feature. This particular one is rated at 12V. To adapt the 12V counter to the 24V requirement (per schematic), simply measure the coil resistance and add an external 2W series resistor equal to this value—not very critical. To use an AC impulse counter, add a 24VDC relay and slave the counter off the relay contacts.
Piezo sensor and charge amplifier
On the charge amplifier, R3 is increased to 47K to take advantage of the higher voltage Vcc. For more information, refer to: http://www.electroschematics.com/7429/bicycle-anti-theft-alarm-circuit/
Charge pump detector
The charge pump detector is essentially the same as a cascade voltage doubler rectifier that is used for signal applications. It detects the peak to peak voltage of the AC input voltage waveform (minus the diode drops). Sensitivity may be potentially doubled simply by replacing D1 & D2 with 1N5818 schottky diodes—note that schottky rectifiers are not specified for low leakage, but D2 requires low leakage—check with ohmmeter before using. Many potentiometer adjustments are provided to adapt the vibration detector to the desired count rate—some tend to be interacting, so it may take patience to set it up for best performance.
Sensitivity, R2 (self-explanatory—controls gain of charge amplifier)
Attack Time, R4 (adding resistance desensitizes detector from very short noise disturbances)
Decay Time, R5 (high resistance adds delay before next disturbance can be counted)
Excessive Vibration Limit, R6 (this is a voltage clamp that prevents the voltage across C3 from getting unreasonably high thus preventing timely recovery for the next count—note that setting it below the threshold will disable counting)
An additional variable is the amount of weight added to the tip of the piezo sensor that affects the SFR (self-resonant frequency). By tuning it to the mechanical resonance of the ‘step plate,’ greater sensitivity may be obtained.
U1 is an open-collector comparator that is applied as a Schmitt trigger. The threshold voltage is established via voltage divider R8 & R9. Positive feedback is applied to the same node via R10 — note the high resistance value. Hysteresis is approx 100mV. U1 has a 2nd unused section — leave terminals floating.
This component is recommended for the serious experimenter.
The recommended power source is 24VDC. This may be obtained from a DC wall wart (approx 100mA). If the wall wart is AC, you must add your own rectifiers and filter capacitor. The 12V supply is derived from 24V via a simple 12V shunt zener regulator (D3 & R11).
Observe that I used a relay in substitution to a counter.