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Teeny Timer: a low-tech
timer for your car
There are plenty of applications in a car where
a simple timer is required. This one doesn’t use
any ICs or even a PC board. It just uses a
transistor, a capacitor, a relay and very little
else. You could wire it up in almost no time at
all and get delays of up to 30 seconds.
By LEO SIMPSON
This simple circuit came about
because one of our readers wanted
a timer for his water-cooled turbo
intercooler. The timer was to be used
to control an electric water pump and
was to operate for a set time (seven seconds) each time after it was switched
off. No doubt you can come up with
a dozen other uses.
Now we could have come up with
a fancier design using an IC such
as a 555 timer. But that would have
required a PC board and this reader
24 Silicon Chip
just doesn’t feel at home with ICs and
PC boards. He also wanted the timer
to operate in the engine bay and that
ruled out consumer versions of the
555 or other timer ICs since their maximum operating temperature is only
70°C.
OK, we thought, “how do we come
up with a simple timer, not using an IC
and the absolute minimum of parts?”
Oh, that was the other requirement: he
didn’t want a lot of parts in it because
he gets confused when soldering them
up! He’s a pretty demanding customer,
this one.
Anyway, we put the feet up on the
desk, thought of faraway places, anything but timers really and finally this
idea popped into the vacuum: “use a
transistor”. The result you can see in
the circuit of Fig.1.
How it works
The heart of the Teeny Timer is a
Darlington NPN transistor, Q1. When
current is fed to its base via the 10kΩ
resistor, the transistor is turned on
and the relay is actuated to operate
whatever you want. The diode across
Top of page: the Teeny Timer uses
one Darlington transistor, a 1000µF
capacitor and not a lot else to provide
delays of about 38 seconds. A bigger
capacitor would give a longer delay.
Note that the circuit is wired on lowtech tagboard.
Fig.1: when switch S1
is closed, the +12V rail
is applied to the 1000µF
capacitor and the 10kΩ
base resistor of the
transistor, to turn the
relay on. When switch
S1 is opened, the 1000µF
capacitor discharges via
the 10kΩ base resistor
to provide a fixed time
delay.
the relay is there to absorb the backEMF generated by the relay when it
turns off.
The circuit operates as follows.
When switch S1 is closed, it feeds
+12V to the 1000µF capacitor and the
10kΩ resistor. This turns on the transistor and operates the relay. When the
switch opens, the 1000µF capacitor
continues to feed bias current to the
transistor and so the relay stays on
until the capacitor is substantially
discharged.
There’s not much more to it than
that. The +12V supply is fed via an
in-line fuse.
Varying the delay
The time delay can be varied by
varying the size of the capacitor. On
our version, the 1000µF capacitor gave
a delay of about 38 seconds. 470µF
would give about 17 seconds; 220µF
about seven seconds; 100µF would
give about four seconds and anything
smaller you wouldn’t bother about.
Longer delays could be obtained by
using bigger capacitors. For example,
2200µF should give about one minute
PARTS LIST
1 12V automotive relay (see
text)
1 SPST toggle switch (S1)
1 inline 3AG fuseholder
1 5A 3AG fuse
1 plastic utility case, 130 x 68 x
42mm
1 BD679 or BD682 NPN Darlington transistor (Q1)
1 1N4004 silicon diode (D1)
1 1000µF 16VW electrolytic capacitor (see text for value)
1 10kΩ 0.25W resistor
1 5-way tagstrip
1 grommet
1 screw and nut to suit relay
Miscellaneous
Spade lug connectors, hook-up
wire, solder.
10 seconds while 4700µF should give
about three minutes.
The actual delays will depend on
the capacitor tolerance, the gain of the
transistor, the ambient temperature
and the supply voltage and whether
you’ve had too much to drink lately.
Not precise enough for you? Hey, this
is a low-tech, low-cost design.
Building it
No PC board! Whoa! Whaddya we
do now?
In a throwback to the dim distant
past, we built the circuit on a 5-way
tagstrip. Fig.2 shows the wiring details. The relay is a standard automotive type with SPST contacts and rated
at 20-30A. They can be purchased
from any automotive parts outlet for
about $11 or from Jaycar Electronics
at $6.95. The relay is mounted with
a single screw and nut, to the base of
the plastic case.
You can either solder the connections directly to the relay or fit the
wires with spade lug connectors, as
we did.
Testing it
This is the easy part. Connect the
circuit up to a battery or 12V supply
and operate the switch. The relay
should operate immediately. Then
turn the switch off. The relay should
stay closed for your desired delay time
and then give a click to show that it
has opened.
We mounted the whole lot in a plastic case and the leads all came in via a
grommeted hole at one end. If you are
going to mount the Teeny Timer in the
engine bay of your car, keep it as far
away from the hot spots as possible.
Mount the case so that the wire entry
hole is at the bottom, so that water
splashes don’t become a problem. SC
Fig.2: the wiring diagram shows all the
details. Note the polarity of the diode and
electrolytic capacitor and make sure you
wire the transistor correctly otherwise it
won’t go.
April 1997 25
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