📄 relay_circuit.htm
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<a name="tuner.gif">
</a><center><h3><a name="tuner.gif">Long Loopstick Antenna</a></h3></center>
<br>Wound on a 3 foot length of PVC pipe, the long loopstick antenna was an
experiment to try to improve AM radio reception without using a long wire
or ground. It works fairly well and greatly improved reception of a weak
station 130 miles away. A longer rod antenna will probably work better if
space allows. The number of turns of wire needed for the loopstick can be
worked out from the single layer, air core inductance formula:
<br><br>
Inductance = (radius^2 * turns^2) / ((9*radius)+(10*length))
<br><br>
where dimensions are in inches and inductance is in microhenrys. The inductance
should be about 230 microhenrys to operate with a standard AM radio tuning
capacitor (33-330 pF). The 3 foot PVC pipe is wound with approximately 500
evenly spaced turns of #24 copper wire which forms an inductor of about 170
microhenrys, but I ended up with a little more (213uH) because the winding
spacing wasn't exactly even. A secondary coil of about 50 turns is wound along the length of
the pipe on top of the primary and then connected to 4 turns of wire wound
directly around the radio. The windings around the radio are orientated so that
the radio's internal antenna rod passes through the external windings. A better
method of coupling would be to wind a few turns directly around the internal
rod antenna inside the radio itself, but you would have to open the radio to do
that. In operation, the antenna should be horizontal to the ground and at right
angles to the direction of the radio station of interest. Tune the radio to a
weak station so you can hear a definite amount of noise, and then tune the
antenna capacitor and rotate the antenna for the best response. The antenna
should also be located away from lamp dimmers, computer monitors and other
devices that cause electrical interference.
<br>
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<a name="cdi.gif">
</a><center><h3><a name="cdi.gif">Capacitor Discharge Ignition Circuit (CDI)</a></h3></center>
<br>
The CDI ignition circuit produces a spark from an ignition coil by
discharging a capacitor across the primary of the coil. A 2uF capacitor is
charged to about 340 volts and the discharge is controlled by an SCR.
A Schmitt trigger oscillator (74C14) and MOSFET (IRF510) are used to drive
the low voltage side of a small (120/12 volt) power transformer and a voltage
doubler arrangement is used on the high voltage side to increase the capacitor
voltage to about 340 volts. A similar Schmitt trigger oscillator is used to
trigger the SCR about 4 times per second. The power supply is gated off during
the discharge time so that the SCR will stop conducting and return to it's
blocking state. The diode connected from the 3904 to pin 9 of the 74C14 causes
the power supply oscillator to stop during discharge time. The circuit draws
only about 200 milliamps from a 12 volt source and delivers almost twice the
normal energy of a conventional ignition circuit. High voltage from the coil is
about 10KV using a 3/8 inch spark gap at normal air temperature and pressure.
Spark rate can be increased to possibly 10 Hertz without losing much spark
intensity, but is limited by the low frequency power transformer and duty cycle
of the oscillator. For faster spark rates, a higher frequency and lower
impedance supply would be required. Note that the ignition coil is not grounded
and presents a shock hazard on all of it's terminals. Use CAUTION when
operating the circuit. An alternate method of connecting the coil is to ground
the (-) terminal and relocate the capacitor between the cathode of the
rectifier diode and the positive coil terminal. The SCR is then placed
between ground and the +340 volt side of the capacitor. This reduces the
shock hazard and is the usual configuration in automotive applications.
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<a name="igniter.gif">
</a><center><h3><a name="igniter.gif">Low Voltage, High Current Time Delay Circuit</a></h3></center>
<p>
In this circuit a LM339 quad voltage comparator is used to generate a
time delay and control a high current output at low voltage. Approximatey
5 amps of current can be obtained using a couple fresh alkaline D batteries.
Three of the comparators are wired in parallel to drive a medium power PNP
transistor (2N2905 or similar) which in turn drives a high current NPN
transistor (TIP35 or similar). The 4th comparator is used to generate a time
delay after the normally closed switch is opened. Two resistors (36K and 62K)
are used as a voltage divider which applies about two-thirds of the battery
voltage to the (+) comparator input, or about 2 volts. The delay time after
the switch is opened will be around one time constant using a 50uF capacitor
and 100K variable resistor, or about (50u * 100K) = 5 seconds. The time can
be reduced by adjusting the resistor to a lower value or using a smaller
capacitor. Longer times can be obtained with a larger resistor or capacitor.
To operate the circuit on higher voltages, the 10 ohm resistor should be
increased proportionally, (4.5 volts = 15 ohms).
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<a name="delay.gif">
</a><center><h3><a name="delay.gif">Power-On Time Delay Relay</a></h3></center>
<br>Here's a power-on time delay relay circuit that takes advantage of the
emitter/base breakdown voltage of an ordinary bi-polar transistor. The
reverse connected emitter/base junction of a 2N3904 transistor is used
as an 8 volt zener diode which creates a higher turn-on voltage for the
Darlington connected transistor pair. Most any bi-polar transistor may be
used, but the zener voltage will vary from about 6 to 9 volts depending on
the particular transistor used. Time delay is roughly 7 seconds using a
47K resistor and 100uF capacitor and can be reduced by reducing the R or
C values. Longer delays can be obtained with a larger capacitor, the
timing resistor probably shouldn't be increased past 47K. The circuit
should work with most any 12 volt DC relay that has a coil resistance of
75 ohms or more. The 10K resistor connected across the supply provides a
discharge path for the capacitor when power is turned off and is not
needed if the power supply already has a bleeder resistor.
<br>
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<a name="relay_i.gif">
</a><center><h3><a name="relay_i.gif">Power-Off Time Delay Relay</a></h3></center>
The two circuits below illustrate opening a relay contact a short
time after the ignition or ligh switch is turned off. The capacitor
is charged and the relay is closed when the voltage at the diode anode
rises to +12 volts. The circuit on the left is a common collector or
emitter follower and has the advantage of one less part since a
resistor is not needed in series with the transistor base. However the
voltage across the relay coil will be two diode drops less than the supply
voltage, or about 11 volts for a 12.5 volt input. The common emitter
configuration on the right offers the advantage of the full supply voltage
across the load for most of the delay time, which makes the relay pull-in
and drop-out voltages less of a concern but requires an extra resistor in
series with transistor base. The common emitter (circuit on the right) is
the better circuit since the series base resistor can be selected
to obtain the desired delay time whereas the capacitor must be selected
for the common collector (or an additional resistor used in parallel with
the capacitor). The time delay for the common emitter will be approximately
3 time constants or 3*R*C. The capacitor/resistor values can be worked out
from the relay coil current and transistor gain. For example a 120 ohm
relay coil will draw 100 mA at 12 volts and assumming a transistor gain of
30, the base current will be 100/30 = 3 mA. The voltage across the resistor
will be the supply voltage minus two diode drops or 12-1.4 = 10.6. The resistor
value will be the voltage/current = 10.6/0.003 = 3533 or about 3.6K. The
capacitor value for a 15 second delay will be 15/3R = 1327 uF. We can
use a standard 1000 uF capacitor and increase the resistor proportionally
to get 15 seconds.
<p>
<img src="RELAY_CIRCUIT_files/relay_i.gif">
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<hr>
<a name="delay10.gif">
</a><center><h3><a name="delay10.gif">9 Second LED Timer and Relay Circuit</a></h3></center>
<br>
This circuit provides a visual 9 second delay using 10 LEDs before
closing a 12 volt relay. When the reset switch is closed, the 4017
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