Need help with component values for power control circuit

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Joined May 14, 2020
I have a simple DC power control circuit that I want triggered by reed relays. It is for model railroad use. One set of reed switches detects the passing of the train . The first switch turns the power on to a trolley car so as the train passes the trolley car is moving. The second reed switch turns off the power as the train passses that point. Then the trolley stops and waits for the next train.

Reed switches 3 and 4 are on the trolley track and when the trolley triggers either, the voltage output is reversed causing the trolley to move in the opposite direction.

The trolley may draw up to 900mA

This is all based on the first diagram on this page

I do not want to complicate this more, as I figure simple is best. I have seen some similar circuits with 2 to 3 tiems the components

I added electrolytic capacitors and diode because load is inductive, and changes polarity and wanted to protect against reverse voltage, and noisy power wupplies.
Addded S5 to have the option of "remembering" or "resetting (forgetting)" the direction that the train last travelled and resuming in same direction.



Joined Jul 11, 2016
R4 must provide enough current for the T3 to conduct your ? 30mA or whatever is specified for the relay (to ensure it does you may set it half the resistance required by the hFE ratio of the T3)

same for the R2 T1 -- only now it has to pass 1A or more required by the track/trolley - using the MOSFET would be more efficient ← takes reworking that part of your "traffic controller"

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