svenschwermer/heng-dimmer

Simple dimmer module designed for the Heng Balance Lamp

Heng Balance Lamp Dimmer

This is a simple dimmer module that is designed for the Heng Balance Lamp that materialized on Kickstarter a while back and is available through various distributors.

Circuit

The circuit was designed with components that I had at hand at the time. Luckily, the lamp is powered via a USB power supply, so I could just pulse width modulate the supply to the lamp in line. The switching transistor is a BUZ11 N-channel MOSFET. It's driven directly by one of the output compare pins of the ATtiny13A.

The user adjusts the PWM duty cycle (i.e. brightness) via a rotary encoder that includes a push button switch. The wires are soldered directly to the underside of the circuit board. Holes allow to install cable ties as strain relief.

The PCB is shared on OSH Park.

Firmware

The program configures the timer to run in fast PWM mode with a prescaler of 8. Since I didn't change the clocking fuses, the microcontroller runs at 1.2MHz. With the prescaler and the 8 bit resolution, the PWM frequency ends up being 586Hz which is sufficient to not see flicker.

7 distinct brightness levels can be selected by turning the rotary encoder. Pushing it brings the lamp to full brightness if it wasn't already. It switches the lamp completely off otherwise. This allows toggling between off and fully on by pushing the button.

The encoder phase and button inputs are sampled every time the timer overflows, i.e. with the same frequency as the PWM frequency. The button is debounced by requiring it to be pushed for > 107ms.

Note: When the JTAGICE3 is attached to the board, the push button functionality doesn't work reliably since the programmer seems to include a pull-up resistor on the SCK line.

Housing

The housing allows the populated circuit board to be fastened by the rotary encoder nut. Unfortunately, the design doesn't allow the PCB to be taken out once the cables are installed. Neither is it possible to access the programming header once the cables are installed and the PCB is installed in the housing.

The big radius on the back side allows to fasten the entire unit to the round base of the lamp using cable ties.

Pictures