■ FIGURE 2. Schematic
for the garage door
wave rectifier. Placing a variable load between PUSH
BUTTON and COMMON, I recorded the voltage and
current as a function of the load resistance (Figure 1). The
garage door opener didn’t trigger until the PUSH
BUTTON voltage was 0.3V or less. The maximum current
of 10 mA available from PUSH BUTTON told me a low
power closer circuit could be powered from that line.
Design and Build
I designed the closer circuit using a CMOS 4060,
14-stage binary counter and oscillator, shown as U2 in
Figure 2. The datasheet gives the formula for the oscillator
frequency as Fosc = 1/( 2. 3*Rt*Ct).
The values shown in the schematic
approximate a frequency of 43 Hz.
A 14-stage binary counter can
count to 16K which means the
most significant bit’s output (Q14)
will go high at count 8192. When
Q14 goes high, it turns on
transistor Q1which grounds the
PUSH BUTTON input to the
automatic garage door opener,
causing the garage door to close.
■ FIGURE 3. Door open sensor
switch with music-wire feeler
mounted on the garage door track.
January 2012 35