Echinos said:
default wrote:
I have set up a one-shot 555 timer to trigger a relay for a few
seconds. With just that set up, it works fine; I can hear the relay
turn on and off. With an LED connected to the relay output, it turns
on and off as expected, along with the relay.
Finally, I hooked up a car fog light that draws 4A, and the light and
relay will turn on, but not off. When I disconnect the fog light from
the relay, then I hear the relay switch off. I can then reconnect the
fog light, and retrigger it again, but the relay again does not
deactivate.
Notes: I have no transistor between the 555 output and the relay. I do
have a 'kickback' diode on the coil side of the relay. Both the 555 and
the fog light are on the same power supply, which happens to be the 12V
output of a PC power supply. The 555 is triggered by a momentary switch
between GND and the trigger pin, and the trigger pin is tied high
through a 100K resistor. The relay is a 12V relay that is rated to
handle 5A on the contact side.
My guess is that the 555 is being retriggered somehow, as if the fog
light is causing a trigger signal to the 555, but I'm not sure why the
200mA of the LED isn't doing anything, but the 4A of the fog light is.
Any ideas?
If the fog light is causing the 555 to trigger (and it might well do
so, on the same supply and all) you should hear the relay click
periodically. Do you?
Yes, I definitely hear the click - on and off when the LED is
connected, but only on when the lamp is connected. I can take the load
off the contact side of the relay totally, and I can hear the relay
turning on and off easily.
What is happening to the power supply voltage while this is going on?
Dunno. I have no o-scope to look at it with, and I have not checked to
see if a volmeter would tell me anything useful yet.
Are the contacts of the relay good for the current? Incandescent
lamps have a very low resistance when the filament is cold, so they
can draw lots of current until they heat - the relay contacts may be
welding themselves shut (a series inrush current limiter would fix
that) for instance a 100 watt bulb may suck 20 amps (>2000 watts) for
a few milliseconds.
The lamp draws 4A when connected directly to the 12V power source, and
the relay is rated for 5A. The relay is still OK, it works fine if I
remove the lamp.
If the light is pulling down the power supply, put a large
electrolytic (>1000 ufd/16 volts) on the 555 circuit and a forward
biased diode just supplying current to the monostable (not the fog
light).
I'll try that. Thanks.
555's need stable power - with no electrical noise to work properly
If this is going in a car - the problem might be better with a low
impedance power supply (battery) or worse - more electrical noise.
This is not going in a car.. Eventually it will be running off a 12V
car battery, however. It will be a circuit used to momentarily turn on
various lights and motors, once I manage to scale up the design after
getting the basics right.
+12 ---+---------------------------------+
a| D2 |
[D1] +----|<----+ |
| .---------. | | | <|
+----|8 555 3|---+---[Ry]---+ |
+| ~ ~ | |
[C1] ~ ~ | [Lamp]
| | 5|---+ | |
| .__1______. | +------+
| | [C2] |
| | | |
Gnd ---+-------+----------+----------+
At a minimum, add D1, C1, C2 and D2 to your circuit
if they are not already there. C1 is a relatively
large electrolytic, say 470 uF (use what you have
on hand > 100 uF, 16V or >). C2 is .01 uF. The diodes
can be 1N400x, 1N4148, 1N914 etc pretty much whatever
you have on hand.
Those parts should be part of the circuit you described,
even if there wasn't a problem.
I do not have D1 or C1 for the VCCbut I have the rest. One other thing
I found is to put a diode between the output and the relay, because
apparently it is possible that without it, the relay can re-trigger the
555 when the relay coil de-energizes. IE:
+12 ---+--------------------------------------+
a| D2 |
[D1] +----|<----+ |
| .---------. | | | <|
+----|8 555 3|-->|---+---[Ry]---+ |
+| ~ ~ | |
[C1] ~ ~ | [Lamp]
| | 5|---+ | |
| .__1______. | +------+
| | [C2] |
| | | |
Gnd ---+-------+----------+----------+----+
I'll try a couple of things and see how it goes.
Another question now.... Once I get this thing operating as required,
what would be the best way to connect this circuit to power, say, 4 of
theses lamps, a small DC motor or two, 10 or so LEDs, and a couple of
other 555's? My initial thought is to use this relay to power the coil
side of other relays, one for each lamp and the motor. For the LEDs and
the 555, I would maybe use 1 relay to supply power to all of them
together.
The last step is to get some sort of sensor that, when tripped, will
turn the whole shebang on for 10 seconds or so, and turn off. The
problem is that it will be outside all year, and in snowy country. The
sensor is to be tripped by a passing scale model train. So, I was
thinking of using an outdoor motion sensor, but it would trip from
passig animals and probably wind making trees move. I've thought of a
sealed limit switch of some sort, or possibly an IR beam with some PVC
tubing to protect the emitter + receiver. Also, I might be able to use
a hall effect sensor of some sort, but I can't think of how to easily
protect such a thing from winter, or even rain in the summer. Maybe I
could bury a vibration sensor underground and seal the thing up?
Cheers
