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Circuits
of the Week - 2012, Jan - June
Last Updated on:
05/01/2012 07:11 AM
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| April 28, 2012 |
Crystal Controlled 1KHz Signal
Generator
A while back I got an email
from a Discover Circuits visitor. He needed a way to calibrate a RPM display for a big
diesel engine. The engine had a tachometer output which produced 12 pulses per rotation
with an amplitude of about 5v RMS. The full scale reading on the engine’s RPM speed
display dial was 5,000. This translates to a pulse frequency of 1KHz at full scale.
To calibrate the display, he needed an accurate 1KHz signal. The 32KHz crystal
controlled oscillator below performs this function. It uses a CD4060 binary counter,
which has internal inverters configured for a classic crystal oscillator. The Q5 output
divides the 32KHz crystal frequency by 32 yielding an output of 1KHz. The output is AC
coupled to the input of the tachometer. |
| Mar 25, 2012 |
12v – 24v AC and DC Day/Night Switch
The circuit uses a bridge rectifier circuit to convert AC into DC. A 12v zener diode
limits the operating voltage of the circuit to 12v. A tiny Schmitt trigger inverter and
a phototransistor detects the ambient light intensity. If there is sufficient light, the
inverter disables a push-pull oscillator circuit. The oscillator, with a differential
output, drives a charge pump circuit which when activated, provides sufficient gate-source
voltage to a pair of n-channel FETs to turn them on fully. The transistor switch can
handle both AC and DC with a current in excess of 5 amps..... |
| Feb 26, 2012 |
Car Ignition 12v Switch
A Discover Circuits visitor needed a way to redirect +12v DC in his car to some auxiliary
loads whenever the car’s engine was running. Many cars have cigarette lighter outlet
jacks, which are either live all the time or become live when the car’s engine is running...... |
| Feb 5, 2012 |
Battery Amp-hour Capacity Tester
I bought some lithium ion rechargeable batteries a while back. I was planning on using them to
modify some solar powered LED spot lights. What I like about these new batteries is that
they are the same size as a standard 1.5v alkaline AA cell. With such a package, I can
use a standard AA battery holder. The battery manufacturer claims an amp-hour capacity
of 2.2 amp-hours. Usually, these amp-hour figures are made using rather small loads,
conducted over a long period of time.... |
| Jan 20,2012 |
Small Pushbutton Controls 10
amps AC
The circuit below uses a pair
of long lasting lithium coil cell batteries to power a latching relay circuit. With each
activation of the small pushbutton, the relay is either latched closed or unlatched open.
Since coin cells can’t provide the necessary peak current, a 500uF capacitor is used to store
the needed energy. The circuit standby current is kept very low by using a stack of five
100uF ceramic capacitors instead of the usual electrolytic cap. An aluminum capacitor,
even a tantalum capacitor leaks too much current, shortening the battery life. By using
the ceramic caps, the overall standby current is only a few microamps...... |
| Jan 8, 2012 |
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..... |
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