DC-Motor Speed Controller

It is common knowledge that Looping Louie is a great game for kids and a great drinking game for adults alike. But for sober players, the game can be rather unchallenging. A solution to this problem is presented in the following article in which fun is increased and time to drunk decreased.


Supply Voltage: 9 VDC
Maximum Current: 1.25 A
Duty Cycle Range: 16 – 72 %


Quad Op-Amp: TL054M BiFET Designed for use with dual power supplies!
Drive Transistor: IRFZ44 N-channel MOSFET


Vpp, idle = 80 mV on the 9 V rail

Potential Problems

On startup, a big 470 uF capacitor causes a big inrush current. On a current-limited laboratory power supply, this makes the supply go into a constant current mode which charges the capacitor linearly. On a cheap wallplug, this could cause its fuse to blow. Potential Solutions:
– Constant Current regulator at the supply input.
– Polyfuse (a heavily temperature-dependent resistor)
– Smaller Cap

Reversing the motor polarity causes a big current surge that might cause the power supply to shut down.Potential Solutions: Non-issue in praxis.


After connecting a USB-PD charger to the type-c port of the trigger board there was no voltage on the power rails. The first suspicion was a faulty trigger board. But after disconnecting the trigger from the circuit and hooking it up to a Rigol DL3021 electronic load up to 3 A could be drawn from it at 9V. Testing for continuity also made sure that there was no open circuit on the rails and also no short between them. Trying a second trigger board of the same type I hooked it up to the electronic load and the circuit in parallel to monitor voltage with no success. The next theory was that there must not be a load connected on startup but even with a constant current load of 1.5 A connected to its output it started up with no problems. Then I started plugging in and unplugging the type-c cable repeatedly. I swapped the cable orientation and suddenly it worked, the was 9 V on the rails. So I concluded (prematurely) that the manufacturer of the trigger failed to implement the type-c line correctly. But after a short rant and break the circuit again failed to power up and I was back to square one. So I tried a different trigger board of a different type. The open circuit voltage of the new trigger checked out but when I connected it to my circuit it also shut down, indicated by its power LED fading. It struck me, the 470 uF capacitor must cause a big inrush current which was a non-issue when I tested it with a laboratory power supply with current limiting but it must cause the board to shut down. Removing the capacitor confirmed this theory and the circuit worked without a hitch. To smooth the power rail a small 10 uF capacitor replaced the previous large one. The pictures below show the supply rail while the motor is connected with the added 10 uF capacitor.

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