Guitar Case Detector Design

A guitar case detector circuit comprises a voltage divider with a 1KΩ resistor in series with a 100kΩ resistor on the circuit board with the junction between the two resistors being fed to an analogue input on an Arduino Nano.

Plugging in a guitar case puts another resistor in parallel with the 100kΩ resistor, hence changing the resistance on the ground side of the voltage divider, as hence changing the voltage at the analogue pin.

The circuit is arranged like this:

Guitar case detector resistor circuit
Case Detector Circuit Design

By using a different value resistor in each guitar case we can detect which guitar case is plugged in.

To help determine the resistors to use, an experiment was undertaken with all the values of resistors I have in stock. One resistor of each value was selected at random, connected into the above circuit, and tested with an Arduino Nano that read and output a range of values of the connected Nano analogue pin. The experiment was repeated with different selection of resistors, this time with an Arduino Uno.

Note that the Nano experiment was conducted in 2021 using a sketch that output a list of all the read values and the range of values was detected by examining the list. The experiment using the Uno was conducted in August 2022 and used a sketch that read 200 values from the analogue pin and output the minimum and maximum values. The sketch was run multiple times and the results were combined. The latter sketch is available from the project GitHub repository, but the former sketch was not saved.


Parallel resistance
The value of the resistor coupled in parallel with the 100kΩ resistor.
Combined resistance
The resistance of parallel combination of the 100kΩ resistor and parallel resistor, rounded to 2 significant digits.
Analogue input reading range
Shows the range of readings read from the Arduino Nano & Uno. Values fall in the range 0..1023, which is the maximum sensitivity of the microcontrollers’ ADC.
Parallel resistance Combined resistance Analogue input
reading range (0-1023)
Nano Uno
Short circuit (0Ω) 0-2 0
10Ω 10Ω 8-10 8-10
22Ω 22Ω 20-22 20-21
47Ω 47Ω 43-48 44-46
100Ω 100Ω 91-94 91-92
150Ω 150Ω 130-135 131-133
200Ω 200Ω 168-169 169-171
220Ω 220Ω 180-184 183-188
270Ω 270Ω 214-217 214-215
330Ω 330Ω 251-253 251-254
470Ω 470Ω 323-325 324-325
510Ω 510Ω 345-347 344-345
680Ω 680Ω 410-411 410-412
1kΩ 990Ω 505 510-512
2kΩ 2.0kΩ 676-679 675-676
2.2kΩ 2.2kΩ 696-697 700-702
3.3kΩ 3.2kΩ 776-779 779-781
4.7kΩ 4.5kΩ 835-839 834-837
5.1kΩ 4.9kΩ 844-849 847-850
6.8kΩ 6.4kΩ 884-888 883-887
10kΩ 9.1kΩ 920-922 921-924
20kΩ 17kΩ 963-968 964-967
47kΩ 32kΩ 988-992 991-994
51kΩ 34kΩ 991-999 993-996
68kΩ 40kΩ 997-1003 996-998
100kΩ 50kΩ 999-1005 1002-1005
220kΩ 69kΩ 1008-1011 1008-1010
300kΩ 75kΩ 1006-1011 1005-1007
470kΩ 82kΩ 1009-1012 1010-1013
680kΩ 87kΩ 1011-1012 1011-1014
1MΩ 90kΩ 1009-1015 1012-1014
Open circuit (∞Ω) 100kΩ 1008-1020 1013-1014

It can be seen that:

When there is no parallel resistance the resistance is 100kΩ. This is taken to mean no guitar case is plugged in.

If we try to support up to six different guitar cases, we need to choose a range of resistances that are sufficiently wide apart to remain distinct, allowing for the normal variation in resistances of the same nominal value, with a generous safety margin added.

The values chosen are:

Resistance Purpose Reading range
∞Ω open circuit, no case connected 958..1023
4.7kΩ case #1 785..885
2kΩ case #2 626..726
1kΩ case #3 455..555
510Ω case #4 295..395
220Ω case #5 130..230
22Ω case #6 0..70

Values that fall outside the reading range are consider invalid and an error should be reported.

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