Project Log #5. DigiGurdy project. 

I finally have a reasonable crank simulator now i.e. a robot gearmotor in reverse (where the crank spins up a motor acting as a dynamo). To create resistance to turning I originally shorted out the motor connections which long term is probably a bad idea as it can cause arcing at the motor brushes and so on. 

Here, to create a more controlled load on my dynamo, I experimented with secondary motor + wind resistance vane, inspired by old clock chime mechanisms, before settling on a simple filament torch bulb (remember those?) as the best low cost compact solution.

To be clear, I could have a large pulley with an adjustable friction band around it, or a wheel with a friction material against its edge. That would cost more, be less compact and also push the assembly and adjustment/fettling time up to get it all working properly. The gearmotor-bulb is low cost, compact and just bolts/glues in place. Also as the bulb flashes nicely with each burst of crank rotation speed which in a hurdygurdy also creates the buzzing rhythm sound, I may be able to make a feature of it.

I am retaining the chopper wheel which senses the speed of rotation of the crank. Originally I had thought I might measure the voltage output of the motor using one of the Arduino analog input pins. However the chopper wheel does have low latency, is working well and the sensor adds almost no extra cost.

Most of the cost of a real hurdy gurdy is the labour of the skilled luthier who builds it. Part of this project is to keep the human assembly time down to an acceptable level to make this an affordable compact second instrument for silent practice. This is why, if I can use or adapt a more or less pre-assembled mechanical mechanism, I will do so where possible.