Microcontrollers come in all sorts and sizes, and it’s very tempting to use them everywhere. If you are in a plan to build your own Vending Machine, probably you need a microcontroller-compatible Drop Sensor circuit.
In a commercial vending machine, its delivery chute uses a drop (vibration) sensor to detect whether product is vended after a selection is made. The drop sensor, attached to the underside of the delivery chute, sends a signal to the micorcontroller when a product hits the delivery chute. As the can or bottle drops onto the product delivery slide, the impact vibration allows the drop sensor to send a logic-level signal to the microcontroller indicating that a product has been vended. After receiving the sensor signal, the micorcontroller will recognize how the machine is programmed and respond accordingly. For example, if the first can or bottle has just vended the control board will stop electric supply to the vend motor at the same time that the impact is registered, thus avoiding a multiple vend.
This design resulted from the need for a replacement of the drop sensor in an old vending machine which is no longer in production and therefore hardly obtainable. The replacement is a homemade module built from components easily found in the workshop of any electronics hobbyist. The circuit diagram is based around the use of the emblematic LM358 IC (IC1-A), worked here as an impact sensor with the help of a standard 27 mm piezo-ceramic element (PZ1).
Test the finished circuit for proper operation by tapping the impact sensor (PZ1). IC1 output (pin 1) should give a short-negative pulse when the sensor is tapped. Note that, it is recommended to route the output of IC1 to the system-micorcontroller through a circuit such as the 555 timer IC-based monoshot (~100 msec) to clean up and/or invert the logic-low level (L) signal output. Finally, put the drop sensor circuit in the hopper of the vending machine, and re-test the drop sensor for proper operation by gently tapping the hopper. Use an alignment screwdriver to turn out the multi-turn potentiometer (P1) clockwise or counterclockwise to increase or decrease the overall sensitivity.
This circuit shows the basic function of a piezo-ceramic element as an impact sensor. Piezoceramic elements are known for what are called the piezoelectric and reverse piezoelectric effects. The piezoelectric effect causes a crystal to produce an electrical potential when it is subjected to mechanical vibration (the reverse piezoelectric effect causes the crystal to produce vibration when it is placed in an electric field). Precision mounting of piezo-ceramic element is essential for good shock/ impact/vibration measurements,and adding mass to the piezo-ceramic element can change its resonant frequency as well as change its baseline sensitivity. Also note that piezo-ceramic elements require some precautions when connecting to sensitive electronic components. First and foremost, the +/- voltage levels created by hard impact can be very high, even around 100V spikes!