Posted By krs on 09/30/2008 10:10 PM
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.
1) Of the literally 2 dozen of these I've made, only a couple have required me to add the back-emf diodes for proper, long-term operations. If it ain't broke, why fix it?
2) Of the literally 2 dozen of these I've made, I've only found it necessary to add the 0.01 cap on pin 5 in one case and that was for an occasional glitch problem. About 8 out of 10 schematics I've reviewed omit these parts too.
3) As I noted, the part values aren't critical. I use parts from cannabilized VCRs, TV sets, DVD players, etc., and Internet surplus sales, so I gets what I gets.
All component values (except the relay which is obviously 6 volts as mentioned) are included in the schematic. If the named part is not in RS stock, use the next "bigger" value and that should do the trick.
These are the circuits that drive my module for the Del Oro Pacific. They use 9 556s and one 555. None have the reverse diodes or 0.01 mfd caps. These drive 10 of the 6 volt relays.