The initial design for the Topsy Turvy Clock had all of the circuits being driven through the regulator on the microcontroller. This is a MIC5219 regulator and is rated at 500mA but also has a very low dropout voltage. The low dropout means I can run the clock from 4 x 1.5v batteries.
I decided to tot up the maximum current draw of each of the components:
- ATmega32U4 max 200.0mA section 29.1 datasheet
- ULM2003 Darlington driver chips leakage current = (100uA + 100uA) x 14 = 2.8mA
- DS1307 Clock chip 1.5mA
- 28BYJ-48 Motor the datasheet is 92mA but people have reported these taking as much as 250mA.
In my testing I’ve not noticed any issue but for the clock running for long periods I don’t want to be running the regulator at (or over) maximum.
So if we add up the worse case scenario for all of these it comes 704.3mA, a lot more than the 500mA that the regulator can provide.
Luckily the driver board provides a solution, there is a jumper on pins 3 and 4 that can be used as a simple switch for the motors. In the modified circuit pin 3 is left disconnected and pin 4 attached to a second supply, in my case direct to the batteries.
This means that the regulator only has to supply the microcontroller, clock chip and Darlington driver chip which should be less than half the rating of the regulator.