E84 Analog Scale Report + Video, Instructions for Setup

IMPORTANT NOTE:

The full pdf of the report accompanying this circuit is attached as a supplementary document, and this pdf contains all the information presented in this article but presented in a more fluid manner. It is highly recommended to read the report pdf.

Introduction

In this report, the team details how to build an analog scale. Scales have seen much commercial use as a way for engineers to measure the mass of objects and people to keep track of their weight, among many other applications. An example digital scale is given in Figure 1. In this project, the team will construct an analog scale, which is different from the digital scale shown here because the signals within an analog circuit are continuous while the signals within a digital circuit are discrete. This allows analog circuits to have a theoretically infinite resolution, which could lead to a greater accuracy when measuring the weight of objects. 

Figure 1: An example digital scale [1]. Scales are used for a plethora of applications, and their impact is widespread. This particular scale is used to keep track of a person’s weight.

The analog scale detailed in this report will measure how many sets of 5 Jenga blocks have been placed on the scale through the use of LEDs, up to a total of 10 LEDs. So, if 0 Jenga blocks are on the analog scale described in this report, 0 LEDs will be on. The maximum number of LEDs that can be on in the system is 10 LEDs, which will happen when 10 sets of 5 Jenga blocks, or 50 Jenga blocks total, are on the scale.

There is also an accompanying video to show the primary components of the analog scale and give a video demonstration of the scale working [6]. https://www.youtube.com/watch?v=kMTFbr8YRqk

System Description

Our system works identically, in principle, to the system of a bathroom scale. Weight, as an input, is applied to the system, and the output of the system is a human-readable indication of how much weight has been applied to the system. For this system, the weight input is sets of 5 Jenga blocks that have been taped together. The human-readable output is the number of LED lights that have been turned on in this system, with the number of LED lights indicating the number of sets of 5 Jenga blocks that have been added to the scale. 

For example, if no Jenga blocks are on the scale, then no LED lights will be on. If one set of 5 Jenga blocks is put on the scale, which is 5 Jenga blocks total, then 1 LED light will be on. If two sets, which is 10 Jenga blocks total, are on the scale, then 2 LED lights will be on. This pattern will continue until 10 sets of 5 Jenga blocks, or 50 Jenga blocks total, is on the scale, at which point all 10 LED lights will be on. This is the maximum number of LED lights that can be on, and adding more Jenga blocks will not turn on more lights. The total system of the Analog Scale is depicted in the block diagram below. 

Figure 2: Full System Block Diagram.

In Figure 2, the input to the system takes the form of sets of 5 Jenga blocks, and the output to the system takes the form of LED lights turning on. For every 5 Jenga blocks added to the system, an additional LED light will turn on up to the maximum of 10 LED lights. 0 LED lights are on if 0 Jenga blocks are on the scale. As a note, the voltage thresholds at which the 10 LEDs turn on are example values, and the exact values are detailed later in this report. 

There are two primary components to this system. The first is that of the load cell, which inputs the Jenga block weight and outputs a voltage on the scale of millivolts. A block diagram describing the load cell is given below. 

Figure 3: Load Cell Block Diagram. 

In Figure 3, the input to the load cell is weight in the form of Jenga blocks, and the output of the system is a voltage difference on the scale of millivolts. Akin to Figure 2, the values given in this diagram are provided as an example and are not the exact values. The exact values are detailed later in this report. 

The second primary component to this system is the analog circuit. The input to the analog circuit system is the voltage difference from the Load Cell, and the output of the analog circuit is the number of LEDs that are on. At the beginning of the analog circuit, the output voltage of the load cell is amplified and placed on a rail. Then, comparators in the form of MCP6002’s are placed such that, when the rail voltage becomes greater than a particular LED’s threshold voltage, the comparator will turn on the LED. Each LED’s threshold voltage is described later in the report. 

The entire circuit diagram for the analog scale is provided below as a simulation via Falstad, for which the link is provided here, https://tinyurl.com/yp9ydyvh. An image of the simulation is provided below. 

Figure 4: Circuit Diagram of the Analog Scale. 

In Figure 4, the left side of the circuit is a simple representation of the load cell, outputting a varying voltage on the scale of millivolts to represent different weight being applied to the system. The rest of the circuit depicts the load cell voltage being amplified by the AD623 onto a rail, and then the LED lights turn on according to whether the rail voltage is greater than or less than each LED’s threshold voltage. The threshold voltages are determined by a simple voltage divider, as described later in the report. 

Component List:

• 1 1kg load cell [2]
• 1 AD623 (Instrumentation amplifier) [4][5]
• 5 MCP6002 (Operational amplifier) [3][5]
• 10 LEDs [5]
• 20 1kΩ resistors [5]
• 1 of each of the following resistors: 8kΩ, 4.3kΩ, 3.8kΩ, 2.7kΩ, 2.2kΩ, 1.6kΩ, 1.2kΩ, 1kΩ, 800Ω, 560Ω, 33Ω [5]
• 2 Breadboards [5]
• Multiple wires [5]

Figure 7. Falstad Circuit Schematic.

The schematic in Figure 7 was then breadboarded. Here are the steps required to build the circuit:

1. Place each op-amp on the board. Only 5 MCP6002s are needed because there are two op-amps in each chip. 
2. Pin 1 to cathode of LEDx
3. Pin 2 to LEDx_threshold_voltage_output (from voltage divider, where x is the current LED circuit that is being built)
4. Pin 3 to output (Pin 6) from AD623 (see Step 2)
5. Pin 4 to GND
6. Pin 5 to output from AD623 (used a rail for easy access that was not being used for power or GND)
7. Pin 6 to LEDx+1_threshold_voltage_output (from voltage divider, where x is the current LED circuit that is being built)
8. Pin 7 to cathode of LEDx+1
9. Pin 8 to 5V
10. Place the AD623 on the board. 
11. Pin 1 to Pin 8 with 33Ω resistor
12. A resistor of 33Ω allowed the output voltage of the in-amp to be about 3V with 50 Jenga blocks. The load cell’s zero point was not actually zero, so we had to add a bias voltage (2V) to the in-amp to force its zero point to be zero. 
13. Pin 2 to load cell output (white)
14. Pin 3 to load cell output (teal)
15. Pin 4 to GND
16. Pin 5 to 2V bias voltage
17. Pin 6 to positive terminal of op-amps (Pins 3 and 5 on each MCP6002)
18. Pin 7 to 5V
19. Pin 8 to Pin 1 with 33Ω resistor
20. Add each op-amp’s voltage divider
21. Pin 3 (and Pin 6, but separate voltage divider) on each op-amp was connected to the node between the voltage divider resistors. 
22. 5V voltage source → variable resistor → (Pin 3/6) → 1kΩ → GND

Once all steps have been completed, the circuit should look like Figure 8, with yellow wires as placeholders for each of the variable resistors from the voltage dividers:

Figure 8. Breadboarded circuit, with placeholder wires (yellow).