Analog Force Meter
The video on the right shows the operation of the analog force meter which I designed for use in model rocket drag/thrust measurements. The amount of force applied to the load cell is indicated by the LED bar graph of which the sensitivity can be adjusted according to a toggle switch.
Specifically, the load cell acts as a Wheatstone bridge, an arrangment of 4 resistances excited by a potential difference. One of the resistances changes in response to a force applied on the load cell. Since that resistance creates a voltage divider with it and the next resistance, we can measure the changing voltage as force is applied to the load cell. By using another, static voltage divider as a reference and a high-gain instrumentation amplifier, we can produce a useful output voltage that is an indicator of force applied to the load cell.
By changing the position of the switch, a different gain resistance can be selected for the instrumentation amp. This effectively changes the sensitivity of the load cell. Resistances can be selected according to the maximum amount of force expected.
The output of the amplifier is connected to a series of eight comparators. To generate the reference for the comparators, a voltage divider is created with 8 different voltages between Vcc and ground. Each comparator is connected to its corresponding node in the voltage divider. In this design, the last comparator before ground activates when given 1/9 Vcc, the second to last activates for 2/9 Vcc, and so on. When a comparator is active, it sinks current from Vcc through the LED attached to it's output. The LED lights and indicates that the force applied is at least that strong, but not strong enough to light to first unlit LED.
