Reversing Controller

Posted By chrisb on 09/30/2008 12:48 PM
I'm interested in building a reversing unit from the schematic in this thread. Is there enough info describing the components to go to RS and buy them?






If you know a bit about electronics - yes.

This schematic is just a basic multivibrator using a 555 timer. You can find that circuit on the net as well as the formula to calculate the values of the capacitor and resistor for any timing interval you want.

I would add a protection diode across the relay coil or the back emf could kill the 555 timer; also the relay has to be sensitive enough to be driven by the 555 timer and the relay contacts have to be robust enough to switch the track current.


All this unit does is switch track polarity at regular intervals, the interval being determined by the setting of the potentiometer.

Yes -

The circuit is extremely basic but I think it will work fine if you set it up correctly.

The 555 timer is configured as a multivibrator with a very low pulse rate - so the relay will switch and change polarity based on the RC time constant of the 200 mf capacitor and the setting of the 500K pot.
That means you can agjust the pulse rate.
The idea is to make the pulse rate a bit longer than the time it takes the train to go from one end to the other - once the loco passes the diode at the end of the track it will stop because the diode will block the current. As soon as the timer reverses the track polarity, the diode will conduct and the train will head back in the opposite direction where the whole thing repeats itsel with the other diode.

So you set the switching time of the unit equal to the time it takes to travel from one end of the track to the other PLUS the time you want the engine/train to stop at each end before reversing and travelling back. Does everything it's supposed to do with a very simple circuit - but of course if the relay switching and thus the reversal happens before the loco reaches the diode section, then the train would reverse immediately which is not exactly good for the engine.

You can get more details here for example:
http://delabs-circuits.com/cirdir/theory/gates/doc00019.html

Rough calculation indicates the switching time of the relay can be adjusted from about 6 seconds to about 75 seconds.

Toddalin -

I must admit I didn't realize that was your circuit - I thoght it was the schematic of the Aristo unit. I just looked at the diagram - didn't read the text.

But as a former commercial designer, I'm curious about a few things.

1. Why do you think a back-emf diode is not required across the relay coil? The 555-type timers I'm familiar with don't provide an internal protection diode and without back-emf protection and the inductive load, the output transistor won't survive for long.

2. Pin 4 and 5 in your circuit are not connected. If not used, pin 4 should be connected to the positive supply and pin 5 should be connected to ground through a 0.01 mf (microfarad) capacitor to eliminate electrical noise.

3. I think a 220 mf capacitor would be more appropriate for the timing capacitor simply because 220 mf is a standard value where as 200 mf is not.

And finally - since it looks as if people want to build this circuit for their own railroad, it would be helpful if you took the time to list the specific components required with the appropriate RS part numbers.

I picked the 1N914 diode and the 78L type 6 volt regulator on purpose to make a point.

A rectifier is not a rectifier is not a rectifier......if they were all the same why do you think there are thousands of different types?

The 1N914 dide can comfortably handle 100ma of current, the 78L type voltage regulator is already maxed out at 100ma - so I went looking for a 5 or 6 volt relay with contacts that can handle 3 amps or better at 24 VDC.
Radio Shack didn't show any on their web site, so I looked at All Electronics, a surplus place. Of the 46 diferent relays they have listed, only one meets the requirements of 3 amp or better DPDT contacts and a 5 or 6 volt coil. This one:
http://www.allelectronics.com/make-a-store/item/RLY-642/6VDC-DPDT-RELAY-PC.-MT./-/1.html

Trouble is that the coil has a resistance of 50 ohms, so the relay will draw 120 ma when operating and you exceed the rating of both the diode and the 6 volt regulator that I listed.

You could try Mouser and Digi-key or active Electronics or Future Electronics or any number of others to try to find a relay with a higher coil resistance, but moving 3+ amp contacts takes a certain amount of energy and you are not going to find a low-current relay that will do that.

My point is simply that you can't just throw a bunch of components together in a circuit and hope that everything will work out in the end.

Maybe you know how to select each one and which component parameters are important and which one are not in a specific circuit, but I'm sure many people who might want to build this unit, cannot.