555 Timer in a car - cleaning up the 12v supply

[Daz]

Hey all, while the results are a success I'm a bit confused as to why, because the 555 is still connected to the noisy supply through the transistor driving the relay. See attached diagram. I'm using a PNP transistor to drive the relay because the circuit was designed to output low (555 timer output is being used as a sink).



As I understand it, when the 555 outputs low, current flows from the noisy supply, into the collector, through the base, through the 555 timer IC and down to earth. So even though I've filtered the supply for the timer circuit, the noisy supply is still going into the 555 pin 3 (output sink) via the PNP transistor.

But the circuit seems to work OK, so the noisy supply that was causing random firing of the 555 timer before doesn't seem to affect it when going into output pin 3 as a sink. Any explanations?

CatalinaWOW, you mentioned that "For a variety of reasons NPNs are the better choice".

Would it be better engineering to use an NPN, and would the NPN completely isolate the noisy supply from the 555 IC? (i.e. with an NPN, is the Collector-Emitter noisy voltage completely isolated from the Base, with no noise transferred through to the circuit driving the base). Thanks.

[Daz]

BTW, thanks SAABfan for the TVS diode suggestion. I've been reading up on those and I'll be interested to learn more about them once I'm happy with this 555 circuit.

TheElectricChicken and tautech, thanks for info on attaching images between text. Got it! :-)

[tautech]

As I understand it, when the 555 outputs low, current flows from the noisy supply, into the collector, through the base, through the 555 timer IC and down to earth. So even though I've filtered the supply for the timer circuit, the noisy supply is still going into the 555 pin 3 (output sink) via the PNP transistor.

 Any explanations?

When the BJT base is pulled low any noise on pin 3 is shunted to circuit GND.

IC suppy noise was the issue and that has been solved for your purpose.

[CatalinaWOW]

Noise through the path you describe doesn't matter much to the 555.  It is the collector of a transistor inside the 555.  The 555 toggles the base of that transistor high and low as the rest of the internal and external circuit operates.  When high the only way for the noise to couple to the rest of the 555 is a tiny capacitance inside the transistor and some parasitic connections that also typically don't operate.  When low pin 3 is effectively shorted to ground so the noise doesn't matter.

ElectricChicken's description of operation is valid, though you might find his description of R2 confusing since he is describing it's operation in the two different forms of the circuit.  His description of R1 as drawn is also relatively correct, though there is a low pass effect (minor) going on.

The preference for NPN in general boils down to being easier to make, which makes them usually cheaper for any comparable set of performance parameters.  Those differences can easily be overwhelmed by the contents of your junk box or the sale at the local electronics outlet.

[TheElectricChicken]

[Daz]

Hi all,

Just to wind up this thread, so everyone knows what I've been talking about, here are some oscilloscope traces of the noisy supply in the car and how the RC filter cleaned things up (it's also practice for me using a scope).

All traces with scope settings 2 volts/div and 20 mS/div

This first pic below shows the car supply with engine OFF but ignition ON and fuel pump ON. The car is from the 1970's and used to have a mechanical fuel pump but this has been upgraded to a modern electronic fuel pump, and I think this fuel pump is responsible for the 25 Hz noise on the supply:





Sometimes this 25Hz fuel pump noise includes some nasty spikes (photo below). I think it was these spikes making the 555 trigger the relay randomly before I put the RC filter in:





The trace below shows the output of the filter circuit (1N4148 on the input and then RC Filter, with R=30 ohms and C=680uF). Again, engine was OFF and ignition and fuel pump were ON. There was no load on the output of the filter when I captured this trace so maybe this trace doesn't prove much?





Now turning the engine ON you can see how the noise on the car supply changes:





And here's the output of the RC filter with Engine ON. Again, when I captured this trace there was no load on the output of the filter, so this might not mean much:

[Daz]

And here are a couple of traces showing how the relay coil affects the supply to the 555 timer circuit. I've gone down to 1 volt/div for these traces to show the voltage drop a bit more accurately, and timebase is now 1 second to show the relay being triggered for the intermittent wiper approx. every 5 seconds.

The first trace shows the effect of having the relay on the filtered (555) supply. You can see how the supply voltage to the 555 circuit drops when the relay is activated for half a second or so. I guess as more current is drawn by the load on the RC filter more voltage is dropped across the series resistor of the RC filter, and the less voltage the 555 circuit gets as its supply. Note that this placement of the relay causes the 555 timer to pulse the relay every 4.6 seconds (quicker than I wanted):






...And for comparison, here's the same circuit out of the car and connected to a clean bench power supply (timing pulse increases to 4.8 seconds with cleaner supply):





The trace below is when the relay has been moved off of the clean 555 supply and on to the noisy car supply. The supply to the 555 is affected a lot less, and subsequently the timing of the 555 circuit is much more accurate (around 5.4 seconds):







...and for comparison, here's the same circuit out of the car and connected to a clean bench power supply (timing pulse increases to 5.5 seconds with cleaner supply):






So in summary: Success! An RC filter to clean the supply to the 555 timer circuit and the addition of a PNP transistor to switch the relay coil on, with the relay coil now connected to the noisy supply so that it doesn't affect the supply to the 555. I've since found some better components to use, so now the diodes are 1N4007, the capacitor in the RC filter is 1000uF 25v, the PNP transistor is a 2N5401 (good for 150v Vce, so definitely should be able to cope with any big spikes on the noisy side of the filter), and the resistor on the base of the 2N5401 to switch it on is 8.2Kohm.

Thanks for all your help,
Daz.

[Daz]

Yep, got it ElectricChicken, thanks. It would be great if it was possible to insert all of the images before posting, but I guess you don't know what image number the server will give to each image until it has been attached.

[Daz]

Thanks Chicken, I guess that helps save space on Dave's server. Can you do one for image 7b that I uploaded? Then they'll all be the same, and I'll put them in my post.

What did you use to correct the trapezoid distortion? I'm using Irfanview but I don't think it can do perspective correction.

[TheElectricChicken]

Can you do one for image 7b that I uploaded?
What did you use to correct the trapezoid distortion? I'm using Irfanview but I don't think it can do perspective correction.

GIMP, it's free and the best I know of. Gimp and blender on linux are the best for drawing that I know of. Irfanview is very cool, but I only know of it for viewing. I'll do that last one. It will be good for the many people who have poor or limited internet connections.

[Daz]

All done, thanks ElectricChicken. I re-uploaded the adjusted images and then realised I could have just linked to yours...  ah well, it is the beginner's forum after all :-)

[TheElectricChicken]

Actually that allowed me to just remove my messages with the resized images. Even before I did that, I DID notice that the page loaded super-fast. I think that will help a good few people who may like to do the same thing. It is a very common problem in cars that gear acts strange because of electrical noise, and this is a lovely guide you've made.

I would like to know if attachments can still be used after the message they are attached to has been removed long ago, maybe some reader knows about this ?

Anyhow, enjoy the next 6 months without a drop of rain now that you've prepared so well. 

[tautech]

Anyhow, enjoy the next 6 months without a drop of rain now that you've prepared so well. 

Good thread Daz, good outcome.