Circuits designed by David Johnson, P.E.
Last Updated on:
Monday, December 25, 2017 02:05 PM
List of Dave's Circuit Designs
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On/off Pushbutton Switch with 30 Minute Timeout
I received an email
from a Discover Circuits visitor. He wanted to control power to a LED camping
light he was building. He wanted to turn the light on and off with a small
pushbutton switch. He also wanted a timer so the light would not stay on longer
than 30 minutes at a time. It seems that his kids often turned lights on when
going to bed and then forgot to turn them off.
The light he was building was to
be powered by 3 big 1.5v D cells, for a 4.5v supply. He was going to use a 3
watt 2.9v white LED
from Cree but drive it with 500ma for a power of only 1.5 watts. He
figured that the batteries would give him about 20 hours of illumination before being
replaced, if his kids didnít sleep with the thing on, thus the need for the 30 minute
circuit I came up with is shown below. A CD4060 is used for the 30 minute
timer. The device has an on-board oscillator and a 14 stage binary counter.
With the components shown the counter completes a half cycle at the Q14 output in
about 1800 seconds.
Transistors Q1 and Q2 form a classic constant current LED driver.
The resistor R7 controls the current Amplitude. I inserted an additional
resistor R6, so the power dissipated in the transistor Q2 was lower.
A2a and A2b schmitt trigger
inverters form a latching circuit. With each activation of the pushbutton, the
circuit either latches the LED on or latches it off. In the on mode, the
counter circuit is allowed to begin counting pulses from the clock oscillator.
After 8192 pulses the output at pin 3 of A1 swings high. This forces the latch
circuit to turn off, which turns off the LED and holds the counter in reset.
on Drawing Below to view PDF version of Schematic