Circuits for DNETCoreGPIO on RPi


As each GPIO pin has an on or off state, it is useful to monitor their states with an LED, regardless of whether the focus is upon them as an individual LED, relay or solenoid, or whether they are part of of cacophony of controls for device such as a motor. To that end, an additional circuit was wired up using an 10 LED Bar Graph. Each LED segment is driven by a Darlington Pair as a current sinking device, in a ULN2003A package (of 7). Each segment is connected to the positive 3.3v rail via a 100 ohm current limiting resistor. That way each RPi GPIO pin is visually monitored.

ULN2003A Solenoid


As previously discuss, the motor is controlled by a H-Bridge implemented using half of a L293D package. The GPIO control pins are Enable, Forward and Reverse. The motor has 2 connections to the package so it can be reversed. The power so supplied is up to 12 volts separate to the RPi supply.

Motor and Relay L293D connections
Motor and Relay L293D connections


The relay is a 12V DPDT 5A Mini Relay. Although it could be configured using the a Darlington Pair, like teh LEDs but straight through connection to the 12v supply rather than the 3.3v rail, use was made of the second half of the L293D. It was connected the same as the motor but hardwired in the forward mode only. Its enable is used to turn the relay off an on. Whilst it could have been used with a more meatier device, for demonstration purpose it was used to control a standalone DC light.


An Ultra Miniature 12V Solenoid was used to drive a small lever arm (like a railway signal) built out of Lego. It only has a small stroke, 8g force at 3mm (130g at 0mm) and so it took a lot of trial and error to get the small stroke amplified enough to have discernable movement of the arm. It is driven from an ULN2003A Darlington Pair sinking 12 Volts. An air solenoid was also tested and worked OK but its quite hard to have an eye-catching demonstration with that.

ULN2003A Solenoid

The RPi Shield, components plus LED Display

The whole box and dice!


The Solenoid Signal video on Youtube

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