The electronic timer circuit in this article has a number of serious problems. As an Electronic Engineer, I would not recommend building the circuit as presented in the article.
While the idea is good, the circuit as presented is far from ideal and potentially hazardous due to the acknowledged overheating problem.
With some simple modifications, the deficiencies can easily be rectified and the operation of the circuit improved at the same time, at the same time using fewer and/or cheaper components.
Overheating
The most serious problem, as mentioned in the text, is that resistors R1 and R2 get quite hot during normal operation. Worst case scenario, if the unit was mounted under the layout in a location with restricted airflow, is that the overheating could cause a fire and destroy the layout and house.
The overheating problem is entirely avoidable, by modifying the circuit slightly.
As designed, R1 and R2 form a voltage divider which charges capacitor C1 to approx. 25% of the supply voltage, i.e. about 4 V. There is no reason not to charge C1 to the full supply voltage, in which case R2 is simply not required. Circuit changes required are:
Change R3 from 470 ohms to 1800 ohms, to adjust for the higher voltage on the capacitor.
Change the capacitor from 10,000 uF 16 V to 2,200 uF, 25 V. This maintains approx the same time constant when discharging.
Remove and discard R2.
The operating time may well be considerably longer, so a smaller value of C1 could be substituted if this is a problem, or alternatively, use a smaller value of R3. If the time is 50% more than required, change either C1 or R3 to a value approx. 2/3 the present value (i.e. 100%/150%). It is probably easier to change the resistor to fine tune the time delay as capacitors are not available in as many different values as resistors.
R1 can remain as designed, as its purpose is simply to limit t
I just got November MR today and was just reading about the timer and had some questions about the circuit. You have answered my questions and provided a better circuit. A new circuit diagram would be helpful, but I can work out what to do from your description.
Someone brought this up in the general forum last week and many of the same concerns were expressed. The whole thing is quite a ‘backwards’ way of accomplishing a time delay. The ‘frontwards’ way would be to have the push button charge the cap and the coil to switch on while the charge leaked away from it. I don’t have a diagram handy but a 555 timer set up for a one-shot would work wonderfully.
I use the Kadee undertrack permanent magnet uncouplers and have hinged them so they drop down when not required. I use a small push-pull control knob on the fascia, which is intended for model aircraft use, but it gives a neat result. The electromagnetic ones are pretty ugly and require a hefty power supply.
PS I am in Perth so just around the corner, so to speak.
I didn’t see the earlier post. With the alterations I suggested, it would work the “frontwards” way you describe.
A 555 timer would be a more elegant solution but would require quite a few extra components. The intent of the MR circuit is not bad in that it does the job, just, with a minimum of components.
I bolded the area of your comment I think is important as to why it was proposed. So many today are used to buying something already made vs making or modeling. Many times I get more info from MRR mags of the 70s to 90s than today, especially with electronics. I appreciate your post very much and just hope more become comfortable with building electronic assemblies which can help their layouts.
BTW, I’m just now trying to figure out the best design for a speedometer with display that doesn’t have to interface with JMRI/Decoder Pro. Any suggestions on where I could find a good one?
I too have some hinged magnets, but bought an electromagnetic one when the Auzzie dollar was high. A 3 amp power supply is pretty hefty and could easily be a source of enough heat to start a fire so I’m going to try your circuit.
cheers
Alan
BTW I think you and I are further apart than any of our mainland US model railroaders