One for the electronics guru's

[BradW]

I am in the process of wiring up a car and the owner wants intermitent wipers. I have searched for a intermitent wiper module out of another car that I could use but thought it might be easier to make one.

I plan to use a 555 timer. I did a basic electronics course about 14 years ago and I have read a bit on the internet but I am still quite new to it all. I have drawn up a basic schematic and plan to make the circuit after the new year but was after a bit of input from more experienced people.

From what I have read the circuit works as follows.

When power is applied C1 charges through R1 and R2. While this is happening the output from pin 3 high. When C1 is charged pin 7 is connected to pin 1 and C1 discharges through R2 and R3. While this is happening the output from pin 3 is low. When C1 is dicharged pin 7 open circuits from pin 1 and the process starts again.

I have used R3 to give a slower discharge and a longer off time. Obviously there will be some testing to get the values of the components to give the required on and off time periods but I have calculated that with the values listed I should get an on time of approx 1.3 sec and an off time of approx 7.6 sec.

I look forward to any feed back on this but please keep it simple. Also if you can post links to any sites with info, that would be good.

[tricky]

In short... 555's suck. They have a large current drain (comparatively), but that isn't really a problem in a car. Advantages are that they are cheap, simple and stable. A more elegant solution would be a variable speed MOSFET switching circuit, although the timer you intend to use is adequate.

The multivibrator circuit you describe sounds allright, but it won't be sufficient. You'll need to use your timer circuit to switch an external relay, because you'll be switching a largish current.

[tricky]

I just noticed you have an image of your circuit. Shall check this out when I have a pinout for a 555 next to me. It's been a long time since I've used one!

[ndgcpr]

I too haven't used one in a while but if you replace R2 with a variable setup it should allow you to have mulitple speeds for intemitten, its a bit late now but when i get some time i will redraw the circuit with what i mean and add some better power control to it.

[thechuckster]

tried Dick Smith or Jarcar for a kit?

years ago i bought a vellemen kit that seemed to do the trick:
<http://www.velleman.be/common/product.Aspx?lan=1&id=9019>
I got it from a Dick Smith store.

[willwal98]

dude if you want something different (and possibly a bit dodgy) use a slow 12v motor (small) with a wheel on it, have a contact on the wheel and as it spins it connects the circuit and up the wipers go (Think distributor). Variable speed too with a pot. I have actually seen this on somethig before, dodge on a car though IMO

[BradW]

Thanks for the input guys.

It will be triggering a relay as the car is getting a full custom wiring harness made from scratch. Very time consuming job.

My first port of call when I was looking into this was Jaycar and Dick Smith to try to get a kit. Neither had anything. I remember seeing a kit many years ago from Dick Smith but no one there knew of it.

I'm not going with a variable sweep time setup as it would mean another switch on the dash somewhere and a set time will do the job fine. The car wont see alot of wet weather use anyway.

Willwal98 Thanks for the input but I think I'll keep trying the electronic way a bit longer .

I look forward to seeing your revised schematics and if you could give a breif explanation on what changes and why, that would be great.

[o_man_ra23]

dude if you want something different (and possibly a bit dodgy) use a slow 12v motor (small) with a wheel on it, have a contact on the wheel and as it spins it connects the circuit and up the wipers go (Think distributor). Variable speed too with a pot. I have actually seen this on somethig before, dodge on a car though IMO

Very dodgy, please dont suggest things like that as somebody may actually use it. Also there are ways to do all that electronically with a 555 timer, or a comparator switching network etc, which will be smaller, cheaper and more stable.

Brad, Make sure you mount all circuit boards with rubber mounts as the vibrations of a car will quickly and easily break your solder joins. The average 555 timer wont have enough current output to drive a relay, so you will need to add a transistor to the output. Im sure Tricky and ndgcpr would have already thought of this, but a MOSFET is quite cheap, uses less current than a BJT(Bipolar Junction Transistor) so it wont heat up as much, and can drive a much larger relay. A comparator input to the MOSFET will help as the fet will then be always fully on or fully off, like a Class D amplifier. This will in turn reduce the current usage of the mosfet, and make it last even longer, and you will also never have a problem with the fet not reaching enough output current to drive the relay, even when the relay gets old and sticky.

Once ndgcpr posts up his schematic ill have a squiz, and see if i can make any suggestions.

Cheers, Owen

[Cool1]

Do they still make the 555??
Just use a pic!

[o_man_ra23]

Not many ppl have a pic programmer, nor do they know how to write assembly code. Paying somebody to do it gets quite expensive. 555 timer is available off the shelf from dick smiths. probably about a buck to 3 bucks. dunno what things are worth retail anymore.

Cheers, Owen

[river]

Hi,

Yes, they still make the 555. You can get a CMOS version also, but really, in the context of things (ie compared to your wiper motor) the standard MOS 555 isn't going to chew the juice, or for that matter you don't need some expensive transistor to drive the relay. You should be able to use the humble 2N2222 or BC108 to drive the relay. The duty cycle will be low and the drain on the trannie won't be high and won't cook. However, if you're worried about this and want a gruntier trannie then go for something like the BD139. It'll drive the relay and won't break a sweat in doing so. You could always use a ULN darlington driver array chip but this is overkill and more expensive than a single trannie.

You could use a PIC, but that's really going over the top for a simple wiper delay - and you have to still have a driver trannie to switch on the relay and you'll have to program the bugger.

With the 555 you can set up a small rotary switch with different value resistors so you can have some set variable delays. Alternatively, you can use a linear pot and make it fully variable.

You can ue a 556 timer, which is just two 555 timers in a single 14pin package, and use one of the 555s to do the wiper motor and the other one to detect when your lights are on (when the ignition is off) and sound a buzzer.

seeyuzz
river

[BradW]

Thanks for the replies guys. Alot of it has gone straight over my head though.
Just need to clear up a few things.

Will the 555 be able to switch a relay or am I going to need a transistor to drive it?

You should be able to use the humble 2N2222 or BC108 to drive the relay.

I take it these are types of transistors?
How are the connected?

I think I have a few more hours of googling ahead of me before I go much further.

[o_man_ra23]

Youve all got me thinkin, maybe i should jam a pic into my car to run the whole car. Or maybe a motorolla chip. Might have to sweet talk the techies at the uni into cuttin me a board... slip em a carton or sumthin. Prototype board is all well and good, but a custom board is soo much neater.

Cheers, Owen

[ndgcpr]

i have always liked the idea of "pic'n" my car. and i have done a few things in my time with the car as well, the next progect5 i am looking at is running a 40x2 LCD off at ATMEGA8 just to tell me stuff like batt voltage and other useful things.

Anyway i have been really busy but just threw this together. the idea of using R# in your circuit Brad is ok but i prefer using as little components as possible . And since we are using a MOSFET or BJT (transistor) if we make it an inverting circuit then we can dispose of the third resistor. I prefer BJT's because i can get them to do exactly what i want (i always have to have a play with MOSFETS to get them to work...

Anyway here you go, i have played with the values to get my software to work, sorry i didn't use actual E24 series so these exact values don't exsist but hey, you get the idea. the RL is the load so the relay or anything. This circuit will work wired from the exisiting switch and assuming there is already a relay to the motor you may just be able to use a decent transistor instead of an extra relay. Anyway we will see how mods to this work.

Oh and below the circuit diag is the simulation of the circuit. the output of the 555 is actually the opposite (long high time, short low time) and then the transistor (PNP) inverts the signal..

[o_man_ra23]

Just check the draw current of the relay. The 2N3906 transistor is only good for 100mA. Also, why is the first pulse on for 1.3s, and the second pulse on for about 0.7 seconds?? What is the on time in steady state?? Im just concerned that the circuit might leave the wipers in odd positions. What happens when the car is switched off halfway through an on cycle?? does it head back to zero when the car is switched back on??

Im thinking there is a relay inside the motor which switches itself off when it returns to the normal position(wipers down), and holds on when not in the normal position, because the wipers always return to the normal position when you switch the car off, and the turn it back on with the wiper switch off. They also return to normal position when you switch the wipers off with the car running. Perhaps the circuit need only run for half a second each cycle, and just adjust the frequency of operation. I think we all need to look closer at the original wiper design, as there should be features there we can use to make the circuitry simpler.

Cheers, Owen