555 timer -> Relay -> Light problem

Hi, Echinos. Here are some things to watch for:

• If you're using an automotive type relay, make sure about the relay pinout. Sometimes, one end of the coil is connected to one of the contacts.

• Driving inductive loads directly from a 555 can be a problem. If the pinout on the relay coil is OK, try using a small transistor to drive the relay, like this (view in fixed font or M$ Notepad):

| | VCC VCC | + + | | | | - C| RY1 | ^ C| | -----. | C| | | | | | | '---o | | | | | | | 3| ___ |/ | o-|___|- -| 2N3904 | 555 | 1K | |>

| | .-. | | | 1K| | | | | | | | | | '-' | | | | | | | === === | | GND GND | | | -----' | (created by AACircuit v1.28.6 beta 04/19/05

You can use a 2N2222, 2N4401 or just about any other small signal NPN transistor. This will probably solve your problem.

Good luck Chris

Posibly the higher load is causing the voltage to ripple some making the timer not work. Try about a 330 uf cap on the 12 supply to the timer. Your pc supply may not be very well regulated under that size load.

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?

What is happening to the power supply voltage while this is going on?

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.

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).

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.

----== Posted via Newsfeeds.Com - Unlimited-Unrestricted-Secure Usenet News==----

The #1 Newsgroup Service in the World! 120,000+ Newsgroups

----= East and West-Coast Server Farms - Total Privacy via Encryption =----

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.

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.

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.

I'll try that. Thanks.

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] +----|). 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.

Ed

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] +----|

I guess I'm going on the assumption that the 555 is sitting there fat dumb and happy until a negative trigger comes in to pin 2 (if my memory is right) then the relay pulls in and light comes on for what should be a short period of time - but just stays on. Have I got that right? Relay and 4 amp light is working on normally open contacts?

What initiates the monostable timing cycle? A switch or some other circuitry?

I dunno about that 5 amp relay. I was referring to a 120 vac 100 watt lamp - reads about 6 ohms until it heats up, then it is over 100 ohms

- halogen lamps are worse in that respect (smaller thicker filaments designed to run hotter).

I really don't understand it not at least clicking - drop out or try to, and the 555 re trigger immediately on the noise. That's what prompts me to ask about the relay.

A good thing to try would be to connect the light - trigger the monostable then physically disconnect the light, while the circuit is powered up and see if the relay drops out.

Depending on the state of your parts supply . . . Why not just eliminate the relay and see what happens? Use a power transistor to control the lamp. With 12 volts and a standard 555 you might have enough drive to saturate a switching transistor directly - you'd sure have enough to turn on a mosfet.

I built a solid state flasher for my motorcycle - can flash from 200 ma to over 10 amps using a single 30 amp mosfet with no heatsink - I cut the tab off the TO220 case to save room.

----== Posted via Newsfeeds.Com - Unlimited-Unrestricted-Secure Usenet News==----

The #1 Newsgroup Service in the World! 120,000+ Newsgroups

----= East and West-Coast Server Farms - Total Privacy via Encryption =----

Correct-a-mundo.

For now, just a momentary pushbutton. Later on it will be some sort of sensor or limit-swith type of thing. All this is on a breadboard atm.

Aha - this is a halogen lamp, I believe. Could be making the problem worse.

Yes, indeed, that's what happens. If I trigger the light, it turns on and stays on, and if I disconnect the lead going from the relay to the light, I hear the relay turn off immediately. I can then re-attach the light, and retrigger it, with the same results.

Basically, I'm working with what I have atm. I have no problem going and getting parts if I need them, but I had seen this described as a way to control higher current from a 555, so it just where I've started. I have tried a power transistor, but it did not seem to work. I think it might have to do with the fact that it is a halogen light. The power transistors I happen to have are the TIP31 and TIP32. I don't think they can handle 4A, but I might be able to use a different lamp with less current.

Impressive. I imagine such a mosfet might be a little pricey?

Cheers

Well, the circuit is now behaving as expected, and the one change that does the trick is the diode on the VCC to the 555. Of course, I think it will be a good idea to have the cap from vcc to ground, too.

Thanks all for the assistance.

Yeah boy, cost a whole 99 cents. Spec on the part is N channel, Vds

50V, Ids 38 amps, Rds (on) .038 ohms.

PN > BUK455-50B

another IRFZ34 50V, 30A, 05 ohms 89 cents

Pretty good lamp drivers - only downside may be that it takes a fast blow fuse to protect it.

I dipped it in varnish and potted it in an empty dental floss case with two wires for the connections. Only way to get a 10 ufd cap to keep up the power while the circuit is busy making a short (shorting its own power supply) was to use a mosfet.

Mounted it to the bike only to find there was an unused ground wire there so it could use a two or three wire flasher - but it was fun to tinker with.

----== Posted via Newsfeeds.Com - Unlimited-Unrestricted-Secure Usenet News==----

The #1 Newsgroup Service in the World! 120,000+ Newsgroups

----= East and West-Coast Server Farms - Total Privacy via Encryption =----

test your supply voltage with the light on.

--
Real Programmers Do things like this.
http://webpages.charter.net/jamie_5

do

and

12V

switch

the

is.

You could use a sealed reed switch for a trigger, with a magnet on the train, or, if the voltage is supplied to the train via the rails and (metal?) wheels, you could use a simple (spring) wire contact that brushes the 0V wheel as it goes past to trigger the '555. ... Fred

do

and

12V

switch

the

is.

Bury a coil, of about the same dimensions as the object to be detected, underground, and amplify the induced current when a metal object passes over it.

R