36-Hour Watchdog Timer - A watchdog timer is often used in computer systems to detect when a computer is hung up and is no longer running properly. If it does become locked up and is not able to supply a reset pulse to the circuit within a defined time frame the timer times out a. . . Circuit by David Johnson P.E.-April, 2010
3-Minute Closet Light - I have a coat closet by my front door. The thing is very dark at night and really needs some kind of illumination inside. I could hire an electrician to install a light in the closet ceiling and then wire it to a toggle switch outside the door but that would cost me quite a bit of money. . . Circuit by David Johnson P.E.-December, 2011
3-Minute Closet Light #2 - The circuit below is powered by three 1.5v alkaline AA cells. With a finger tap to the pushbutton trigger switch, a cluster of 6 wide angle white LEDs is turned on. The lights remain on for about 3 minutes, then will turn off. The circuit’s standby current is only a few microamps. A set of fresh batteries should last at least 200 light applications. The circuit uses a Schmitt trigger inverter and two transistors. When the pushbutton switch S1 is pressed, the 10uF capacitor C1 is discharged. . . Circuit by David Johnson P.E.-November, 2010
555 Timer Forms Simple Circuit PWM Motor Controller - Using a CMOS
version of the 555 timer, this circuit can be used to control the speed of a motor by adjusting the duty cycle of the pulses sent to the motor. . . Circuit by Dave Johnson P.E.-February, 2007
60hz Clock Generator is Line Powered - This circuit is connected to the 120vac power line and transfers 60Hz clock pulses to a logic circuit. The optoisolator used provides 5000 volts of isolation between the power line and the logic side of the circuit . . . Hobby Circuit designed by Dave Johnson P.E.-February, 1999
Accurate One Shot Pulse Generator - This timer circuit draws only 3ua of current, and can be powered by DC supplies ranging from 3v to 15v. The circuit produces accurate pulses whose time can easily be adjusted, based on the R1 x C1 time constant. The time should not change,. . . Circuit by David Johnson P.E.-September, 2010
Automatic Timeout included with this On/Off Flip/flop Circuit - This circuit is ideal when a device needs to be turned on and off with a single pushbutton switch, but also needs to turn itself off after some period of time. With the components shown, the output will stay on for only about 20 seconds . . . Hobby Circuit designed by Dave Johnson P.E.-May, 2012 |