# Dog Weight Scale Part 7: Choosing Matching Resistors

In my previous post, I worked through the calculations of weight and center of gravity when using four Load Cell Amplifiers instead of one.  In this post, I build the circuit for the first of the four Load Sensor / Load Cell Amplifier combinations I’ll be using.

The Sparkfun Load Cell Amplifier is designed to connect a Load Cell to an Arduino.  A Load Cell contains a full (4 resistor) Wheatstone Bridge, but a Load Sensor contains only half of a Wheatstone bridge.  To connect a Load Sensor to a Load Cell Amplifier, I need to add two resistors: R3 and R4 in the following diagram.  The dotted box represents the Load Sensor. The triangle in the middle of the diagram represents the Load Cell Amplifier.  As the weight on the Load Sensor increases, R1 decreases, which causes the voltage V1 to increase, causing the digitized amplifier output to increase.

# DIY Glockenspiel: the circuit

Earlier, I described how to build a frame for the glockenspiel. In this post, I cover the circuit that will strike each chime.

I’ve successfully tested the glockenspiel control circuit. It’s an Arduino Mega 2560, a Sparkfun Wifi Shield,  and 19 repetitions of a simple solenoid control circuit.

(by the way, the solenoids in the photo are there only for testing. In the finished Glockenspiel, there will be long wires connecting the circuit to the glockenspiel-mounted solenoids.)

I started from a drum control circuit described in Make Magazine.

Then I chose a few specifics: at TIP120 transistor, a 1000uF capacitor, a 1N4004 diode, and a 5V solenoid from Sparkfun. …and did 19 iterations of the circuit; one per chime in the glockenspiel.

I’ve done a quick Fritzing circuit diagram, which I plan to upload to GitHub.

My main concern was whether the solenoids could be powered only by the VIn pin on the Arduino when powered via a 9V 650mA wall power supply. Turns out the answer seems to be “Yes”: the 1000uF capacitor prevents the solenoid from drawing down the main power enough to reset the Arduino, and the power consumption of the solenoids (see below) is easily supported by the power supply.

To avoid burning out the solenoids each one is powered only a few milliseconds at a time.  This causes the solenoid to hit the chime with just enough power to ring nicely, but not too loudly. The math’s pretty straightforward: the data sheet provides the maximum average wattage that the solenoid can dissipate and the minimum resistance of the solenoid. The power supply puts out about 9V. Working out the numbers, it looks like for quickly-paced music, a 6ms-per-strike is about the maximum the solenoid should support (conservatively). Playing most music, this shouldn’t be anywhere near the maximum power the solenoid can support.

You can see a video of the test of this circuit, showing that it can powerup, power down, and run through each of the solenoids.