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From ddixon@ES.COM Thu Nov 21 16:54:58 1996
To: ahicks@nmt.edu
Subject: Short Interval Timer Circuit
Hi Aaron,
A while back you added an article that I wrote on "flasking with home-made
equipment" to your FAQ under flasking.stem-prop. In that article, I mention
a 40 second timer circuit. A number of people have contacted me to get the
circuit description, so I thought you might also want to add it to your FAQ.
Feel free to use it if you would like.
Darrell (ddixon@es.com)
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40 SECOND TIMER CIRCUIT
21 November, 1996
Darrell K. Dixon
email: ddixon@es.com
INTRODUCTION:
The conditions under which I grow my orchid seedlings, require a short
daily misting. Since commercially available short interval timers are
hard to find, I designed the following timer circuit. Building it
requires a minimal knowledge of electronic circuitry and components.
CIRCUIT DESCRIPTION:
The circuit is built abound a 555 type timer chip. When power is initially
applied to the circuit, the output from the chip goes high for the time
interval specified by the RC network, (which I have set to about 40 sec).
After this interval, the output goes low and stays low until power is
removed from the circuit and re-applied. The output from the circuit
drives a relay which drives a solenoid valve. Power to the circuit is
supplied by an ordinary appliance timer, like the ones that plug into the
wall with a lamp plugged into them.
So, when the appliance timer applies power to the timer circuit, the timer
circuit applies power to the relay which activates the solenoid valve. The
timer circuit removes power from the relay after the time period specified
by the RC network, even though the appliance timer is still applying power
to the circuit.
The appliance timer could have multiple on/off settings so that watering
could be done at multiple times during the day.
PARTS LIST:
. Capacitor, C1 (6.5 microfarads):
The capacitance part of the RC network.
. Diode, D1 (1N4003):
Prevents the relay coil's back-EMF from damaging transistor T1.
. Power Supply, 12 volts DC:
This power supply plugs into the appliance timer and provides Vcc+ for
the circuit.
. Relay, 12 volt DC coil:
Drives the solenoid valve.
. Resistor, R1 (6.2 megohms):
The resistance part of the RC network.
. Resistor, R2 (2.2 K-ohms):
Input to the gate of transistor T1.
. Timer, Appliance type:
Applies power to the 12 volt DC power supply.
. Timer Chip, 555 type:
This is the heart of the timer circuit.
. Transistor, T1 (2N3053):
Drives the relay.
TIME PERIOD:
The time period of the circuit in seconds is approximately equal to the
product of R1 in megohms and C1 in microfarads. Hence, for the values
specified: 6.2 x 6.5 = 40.3 seconds.
CIRCUIT LAYOUT:
. CAPACITOR C1:
Connect the positive side of C1 to one lead of R1. Connect the
negative side of C1 to ground.
. DIODE D1:
Connect the two leads of D1 to the two leads of the relay coil.
(Putting D1 in parallel with the coil.) Connect the cathode lead
of D1 to Vcc+. Connect the anode lead of D1 to the drain of T1.
With the cathode end of D1 connected to Vcc+, you may think that
D1 is connected incorrectly. It is not. The purpose of D1 is to short
out the relay coil when power is removed from the coil, thus preventing
the coil's back-EMF from damaging T1.
. RELAY:
The coil leads are already connected to the two leads of D1.
. RESISTOR R1:
One lead of R1 is already connected to the positive side of C1.
Connect the other lead of R1 to Vcc+.
. RESISTOR R2:
Connect one lead of R2 to pin 3 of the timer chip. Connect the
other lead of R2 to the gate of T1.
. TIMER CHIP:
. Pin 1: Connect to ground.
. Pin 2: Connect to pin 6 and to the junction between C1 and R1.
. Pin 3: Already connected to one lead of R2.
. Pin 4: Connect to Vcc+.
. Pin 5: Not used.
. Pin 6: Already connected to pin 2.
. Pin 7: not used.
. Pin 8: Connect to Vcc+
. TRANSISTOR T1:
. Source: Connect to ground.
. Gate: Already connected to one lead of R2.
. Drain: Already connected to the anode side of D1.
CONNECTING A SOLENOID VALVE TO THE CIRCUIT:
To drive a solenoid valve with the circuit, simply use the relay contacts
as a switch for the solenoid valve's power supply.
COMBINING THE SOLENOID VALVE'S POWER SUPPLY AND THE 12 VOLT DC POWER SUPPLY
Instead of driving a 12 volt DC power supply with the appliance timer,
drive a transformer that has an output voltage equal to what the solenoid
valve needs. The output from this transformer goes two places: through the
relay contacts to drive the solenoid valve, and to a 12 volt DC power
supply circuit. This power supply circuit is simply a full wave bridge
rectifier chip, a 12 volt regulator chip and a filter capacitor. Vcc+ is
the output from this power supply circuit.
CASCADING TIMER CIRCUITS:
I believe that multiple timer circuit, each with its own relay, could be
cascaded. This way, as one turns off, the next one would turn on. This
would be the way to drive multiple solenoid valves if water pressure is a
problem.
To cascade the circuits, I would use a DPDT relay. One output from the
relay would drive the solenoid valve. The second output from the relay
would drive the next timing circuit. This second output would get its
power directly from the 12 volt DC power supply. Its contacts would be
open when the first output was driving the solenoid valve and then closed
when the circuit's RC time constant had expired, thus enabling the next
timer circuit.
MAKING A VARIABLE TIME PERIOD:
To make the time period of the circuit variable, use some type of variable
resistor in the RC network.
LONGER TIME PERIODS:
Since the time period is determined by the resistor and capacitor from
the RC network, larger values will give longer periods of time. However,
there is a limit to how large these values can be and still have the
circuit function dependably. Also, you will probably have trouble finding
components that will give a very long time period. Anything much over 90
seconds is probably beyond the capacity of this circuit.
There are other circuits and timer chips that will give longer periods of
time. Books are available that cover these other chips and circuits. Look
specifically for books on TIMERS and TIMER CIRCUITS.
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