Light Dimmer 2

• electronics

So last week I got the dimmer working with a potentiometer controlling the delay of a 555 timer (which in turn controlled the brightness). I thought replacing this potentiometer with a microprocessor would be trivial, but it ended up requiring a little bit of work. Here’s what happened.

The analogWrite() function of the Arduino outputs PWM. This can be converted to an analog signal with a simple RC low-pass filter. The larger the time constant R*C of this filter, the less ripple there will be in the output signal, but also the longer it will take the signal to change to a new value (as the capacitor will charge more slowly). The PWM frequency of of the ATMega328P is 500Hz, so

Unfortunately, you can’t connrct this voltage to the capacitor directly; the timer simply won’t work for two reasons. The first is because the output impedance of the microprocessor is low enough that it charges the capacitor almost instantaneously. The other problem is that the capacitor voltage can only get up to this signal level. That means if the PWM signal is less than the THRESH level of the timer (2/3 Vcc), the timer won’t trigger.

One thing people do to control the timer with a voltage is adjust the control voltage rather than the time constant of the RC network. By pulling the control voltage lower, the RC network will reach that level sooner and trigger faster (and naturally rasing it has the opposite effect). However,

What I really need is something where looking up from the capacitor it appears to be a 5V source with an output impedance between 0 and 10K ohms. This output impedance needs to be voltage controllable. One option is a digital potentiometer. This may be the best way to go moving forward, but for the weekend, I needed some analog way of accomplishing this.