# Switching Circuit from the 80's ?



## T-Man (May 16, 2008)

I have a bank of four of these modules. They were acquired from the LHS and ended up with me after it closed.

So I am writing out the circuit diagram and performing a little reverse engineering on how this thing works.






















So the center is a 555 Timer (Developed in 1972). The resistors left and right are 100 ohm and the standing resistor is 850 ohm attached to the base of the transistor 2N94A. A NPN transistor from the 50's.The Sigma Relay is still being sold today. The nut under the relay holds it in place. The relay poles EFGH I can figure out later. F& G are connected to the center poles. 
The posts above and below the %%Timer are the positive and negative 12 volt power with the gray wires.

C & D are posts connecting the Timer to the positive terminal by 100 ohm resistors.
A & B are curious wire connections. AC and BD must activate the relay.

A curious fact about the 555 Timer is that connections 5,6,7 are not being used.


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## gunrunnerjohn (Nov 10, 2010)

Well, the 555 is from 1982, at least that chip. 










I'm not sure if that board serves a useful function.


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## T-Man (May 16, 2008)

Well John that could be right. The boards look like they were used with all the wires. 
I am still going to try to find a circuit with the timer and a transistor.
Thanks for date of the Chip.


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## DonR (Oct 18, 2012)

That circuit board doesn't seem to serve any function on
a typical model railroad layout. Trains today are in 4 main
scales plus a number of lesser scales. Most use either
a variable voltage, polarity reversing DC of 0 to 14 or so volts
on the track put out by a device we call a 'power pack'
that controls one train at a time...

Or the more modern DCC (digital command control) which puts
14 volts modified AC on the track along with digital control
information that enables the operator to individually control 3, 4 or
more DCC decoder equipped trains running at the same time. 

There is the possibility that your devices could
have been used for some sort of detection or semi automatic 
operation of accessories but it is totally unfamiliar.

Don


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## T-Man (May 16, 2008)

Thanks Don. I am going with an 80's mindset. LEDs were getting popular. The Sound of Steam was getting replaced by electronic boards. The purpose cannot be too complicated. I have not looked at an HO switch in years but suspected it to operate a switch. It is an overkill if used for a standard two coil switch that only needs a momentary switch. After reading your post I thought of signals. Move the switch and activate the signal. My next step is to figure out the relay. Attach some lights.

John is does work. With 12 volts DC applied the relays latched. The position is controlled by the AC and DC terminals. With clear cases I can see the relays responding. One of three modules tested Two worked the third has a broken wire.

On second thought the switch control is impractical because the relay sustains power and is not momentary. BUT it may be practical for controlling reverse loops. Then four reverse loops on a table?? Maybe too many. At least there would be four blocks.

So far the operation is two momentary switches controlling a DPDT relay. 

The 555 Timer is a puzzle. Reviewing some books and magazines and it's like the plan is to use only half the chip.


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## DonR (Oct 18, 2012)

Have you seen any indication of the devices maker?
There is the possibility that you have a treasured
and classic, genuine Rube Goldberg design. 😉

Don


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## T-Man (May 16, 2008)

All I know is the boards have stamped letters. Holes were custom drilled. The ends are plastic screwed to a metal standoff. No markings from what I can see.


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## MichaelE (Mar 7, 2018)

Might have been used as a primitive block signaling relay board.


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## gregc (Apr 25, 2015)

does this look correct?
guessing the "can" on the left is an npn transistor
there's a wire between it an resistor that may go to pin2


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## T-Man (May 16, 2008)

It looks good. !K resistor is off the transistor and the other two are 100 ohms. A wire is off the IC 3 and B is off the 3 side of the transistor.


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## gregc (Apr 25, 2015)

looks like the 555 is used as a bistable latch to hold the relay in particular state


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