relay_circuit.htm

FM MUDULATION SYSTEM COMUNICATION

decade counter will be reset to the 0 count which illuminates the
LED driven from pin 3. The 555 timer output at pin 3 will be high and
the voltage at pins 6 and 2 of the timer will be a little less than
the lower trigger point, or about 3 volts. When the switch is
opened, the transistor in parallel with the timing capacitor (22uF)
is shut off allowing the capacitor to begin charging and the 555
timer circuit to produce an approximate 1 second clock signal to the decade
counter. The counter advances on each positive going change at pin 14 and
is enabled with pin 13 terminated low. When the 9th count is reached, pin
11 and 13 will be high, stopping the counter and energizing the relay.
Longer delay times can be obtained with a larger capacitor or larger
resistor at pins 2 and 6 of the 555 timer.

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<a name="cnt_down.gif">
</a><center><h3><a name="cnt_down.gif">9 Second Digital Readout Countdown Timer</a></h3></center>
<br>

This circuit provides a visual 9 second delay using a 7 segment digital
readout LED. When the switch is closed, the CD4010 up/down counter is
preset to 9 and the 555 timer is disabled with the output held high.
When the switch is opened, the timer produces an approximate 1 second
clock signal, decrementing the counter until the 0 count is reached.
When the zero count is reached, the 'carry out' signal at pin 7 of the
counter moves low, energizing the 12 volt relay and stopping the clock
with a low signal on the reset line (pin 4). The relay will remain energized
until the switch is again closed, resetting the counter to 9. The 1 second
clock signal from the 555 timer can be adjusted slightly longer or shorter
by increasing or decreasing the resistor value at pin 3 of the timer.
<p>

The CD4510 is a CMOS Presettable BCD Up/Down counter which can
be preset to any number between 0 and 9 with a high level on the
PRESET ENABLE line, (pin 1) or reset to 0 with a high level on the
RESET line (pin 9). Inputs for presetting the counter (P1, P2, P3, P4)
are on pins (4, 12, 13, 3) respectively. The counter advances up
or down on each positive-going clock transition (pin 15) and the
counting direction (up or down) is controlled by the logic level
on the UP/DOWN input (pin 10, high=up, low=down). The CARRY-IN signal
(pin 5) disables the counter with a high logic level.
</p><p>
The CD4511 is a CMOS BCD to 7 segment latch decoder capable of sourcing
up to 25 mA which allows it to drive LEDs and other displays directly.
A LATCH-ENABLE line (pin 5, active low) stores data from the BCD input
lines. A LAMP-TEST input (pin 3, active low) can be used to illuminate
all 7 segments, and a BLANKING input (pin 4, active low) can be used to
turn all segments off. The LED display must be a common cathode type so
that the segments are illuminated with a positive voltage on their
respective connections.

Complete data sheets for the CD4510 and CD4511 can be obtained by
answer fax from <br>

<a href="http://www.semi.harris.com/search.htm">Harris Semiconductors (search)</a>
<br>


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<a name="therm.gif">
</a><center><h3><a name="therm.gif">Electronic Thermostat and Relay Circuit</a></h3></center>
<br>
Here is a simple thermostat circuit that can be used to control a
relay and supply power to a small space heater through the
relay contacts. The relay contacts should be rated above the
current requirements for the heater.
<br><br>
Temperature changes are detected by a (1.7K @ 70F) thermistor placed
in series with a 5K potentiometer which produces about 50 millivolts
per degree F at the input of the LM339 voltage comparator. The two 1K
resistors connected to pin 7 set the reference voltage at half the
supply voltage and the hysteresis range to about 3 degrees or 150
millivolts. The hysteresis range (temperature range where the relay
engages and disengages) can be adjusted with the 10K resistor between
pins 1 and 7. A higher value will narrow the range.

<br><br>
In operation, the series resistor is adjusted so that the relay just
toggles off at the desired temperature. A three degree drop in
temperature should cause the relay to toggle back on and remain
on until the temperature again rises to the preset level.
The relay action can be reversed so it toggles off at the lower end
of the range by reversing the locations of the 5K potentiometer and thermistor.
The 5.1 volt zener diode regulates the circuit voltage so that small
changes in the 12 volt supply will not effect operation. The voltage across
the thermistor should be half the supply or about 2.6 volts when the
temperature is within the 3 degree range set by the potentiometer.
Most any thermistor can be used, but the resistance should be above
1K ohm at the temperature of interest. The series resistor selected
should be about twice the resistance of the thermistor so the adjustment
ends up near the center of the control.

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<a name="therm1.gif">
</a><center><h3><a name="therm1.gif">Thermostat for 1KW Space Heater (SCR controlled)</a></h3></center>
<br>
Below is a thermostat circuit I recently built to control a 1300 watt space heater.
The heater element (not shown) is connected in series with two back to back 16 amp
SCRs (not shown) which are controlled with a small pulse transformer. The pulse
transformer has 3 identical windings, two of which are used to supply trigger
pulses to the SCRs, and the third winding is connected to a PNP transistor pair
that alternately supply pulses to the transformer at the beginning of each AC
half cycle. The trigger pulses are applied to both SCRs near the beginning of
each AC half cycle but only one conducts depending on the AC polarity.
<p>
DC power for the circuit is shown in the lower left section of the drawing
and uses a 1.25uF, 400 volt non-polarized capacitor to obtain about 50mA
of current from the AC line. The current is rectified by 2 diodes and used to
charge a couple larger low voltage capacitors (3300uF) which provide about
6 volts DC for the circuit. The DC voltage is regulated by the 6.2 volt zener
and the 150 ohm resistor in series with the line limits the surge current when
power is first applied.
</p><p>
The lower comparator (output at pin 13) serves as a zero crossing detector
and produces a 60 Hz square wave in phase with the AC line. The phase is
shifted slightly by the 0.33 uF, 220K and 1K network so that the SCR
trigger pulse arrives when the line voltage is a few volts above or below
zero. The SCRs will not trigger at exactly zero since there will be no
voltage to maintain conduction.
</p><p>
The upper two comparators operate in same manner as described in the
"Electronic thermostat and relay" circuit. A low level at pin 2 is
produced when the temperature is above the desired level and inhibits
the square wave at pin 13 and prevents triggering of the SCRs. When
the temperature drops below the desired level, pin 2 will move to an
open circuit condition allowing the square wave at pin 13 to trigger
the SCRs.
</p><p>
The comparator near the center of the drawing (pins 8,9,14) is used
to allow the heater to be manually run for a few minutes and automatically
shut off. A momentary toggle switch (shown connected to a 51 ohm resistor)
is used to discharge the 1000uF capacitor so that pin 2 of the upper
comparator moves to a open circuit state allowing the 60 Hz square wave
to trigger the SCRs and power the heater. When the capacitor reaches
about 4 volts the circuit returns to normal operation where the
thermistor controls the operation. The momentary switch can also be
toggled so that the capacitor charges above 4 volts and shuts off the
heater if the temperature is above the setting of the pot.
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