Background Scale-model road systems such as Faller https://www.faller.de utilise vehicles that follow a ferromagnetic wire embedded in the road surface. The vehicles operate at full speed and continue until the on-board rechargeable battery runs out. There is limited remote control. At strategic places an electromagnet under the track can operate an on/off sensor in the vehicle.

Problem This on/off behaviour - for example at a bus stop or road junction brings the vehicle to an abrupt and very unrealistic stop. The wheels are driven via a worm (screw) gear so the wheels lock when the motor stops. Video showing abrupt stop and start https://youtu.be/emejjaj3c6E?t=411

Question More complex and expensive digital systems are commercially available that simulate realistic acceleration and braking behaviour. Nevertheless, can I make a simple analog circuit to simulate inertia in these tiny vehicles?

My naive solution I would like to put a capacitor in series with a resistor across the motor terminals. When battery power is switched off, the capacitor continues to power the motor until it is discharged. Likewise, on switching on the current, the motor starts slowly whilst the capacitor is charging. The component values can be adjusted to give realistic artificial inertia according to the size of the vehicle, e.g. Lorries will take longer to stop than cars.

Envisaged problems Components have to be small (tiny) to fit on an HO (1:87) scale model. I think that I will need a huge capacitor to power the motor as it discharges. Is my idea sound - even in principle - or there another simple solution?

NOTE These vehicles are only required to go in a forward direction - no reversing needed.

Many thanks.

P.S. I'm thinking that by using a power transistor (op amp?) to detect the on/off switch, I could reduce the size of the capacitor and raise the value of the resistor by applying them to the input of the op amp. Haven't thought this through yet.

EDIT Technical specs found so far:-

(a) I measured resistance across the motor terminals - 1 ohm

(b) The battery is a Varta V 150 H; Rechargeable Ni-MH Button; 1.2 V

(c) I was able to measure the current - 28 mA

(d) I found a circuit diagram (unofficial) http://simgw.cosmos.ehime-u.ac.jp/~shimizu/fallercar1/carsys032.jpg

EDIT Electromagnets are mounted under the track at strategic points where a stop may be required, The vehicle is steered by a permanent magnet that follows the under-track passive ferromagnetic wire. There is a reed switch inside the vehicle that detects the on-or-off-ness of the electromagnet and disconnects the battery from the motor accordingly.

  • \$\begingroup\$ Battery / motor voltage would be useful as would motor resistance and normal running current measurement. Add all the details into your question rather than in the comments. \$\endgroup\$
    – Transistor
    Oct 3, 2018 at 14:55
  • \$\begingroup\$ @Transistor - Thanks for your reply. I'm somewhat new to all this so I'll have to research the answers. I'll come back later. In the meantime, I've added a postscript that I think might go some way to solving the problem of capacitor size. \$\endgroup\$ Oct 3, 2018 at 15:35
  • \$\begingroup\$ Find a multimeter. That's all you'll need to take the measurements. \$\endgroup\$
    – Transistor
    Oct 3, 2018 at 15:45
  • 1
    \$\begingroup\$ Just disconnect the battery for the current measurement, and connect with wires and insert the multimeter on mA range in series with one of them. Don't forget to check the plugs are in the right sockets before measuring current and before taking your next voltage reading. \$\endgroup\$
    – Transistor
    Oct 3, 2018 at 16:36
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    \$\begingroup\$ Judging from the performance in the video to significantly improve the aesthetics of the way the vehicles drive, you would want to find or design a tiny drive controller for them. If you made them slow down proportionally to how fast they are turning(how far their wheels are turned at the moment) it would greatly improve the naturalness of the look. You may potentially have to PWM drive the motor. It is also not clear how the electromagnets work or how they are positioned in the vehicle, nor how much free space might be available for circuitry. \$\endgroup\$
    – K H
    Oct 4, 2018 at 7:37

2 Answers 2


Would less than 50 cents be cheap enough? Then you could program a small microcontroller to simulate the velocity profiles you seek, and get a better result. Otherwise, with due respect, this is an XY problem if you’re only willing to consider analog. ATtiny is available in an SOT package.


Adding an RC filter to the circuit could help it simulate inertia, assuming that you applied the capacitor to the right part of the circuit. This picture shows what a capacitor does to a step response (which would be starting then stopping).

enter image description here
Source: http://www.dsplog.com/2007/12/02/digital-implementation-of-rc-low-pass-filter/

The caveat would be you may need a very large capacitor, bigger than your model depending on what kind of currents the motor uses. Start with small capacitors 1uF or 10uF across the motor then work your way up.

  • \$\begingroup\$ Thanks. If I'm reading that graph correctly then I'm not sure it will serve the purpose. It looks to me that the current rises very sharply initially which presumably will still give a jerky start. Maybe, as you suggest, I'll just have to suck it and see with different value capacitors. \$\endgroup\$ Oct 3, 2018 at 23:08
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    \$\begingroup\$ That graph represents the voltage across the motor, which could be a problem if your motor does not have a linear voltage to torque conversion. \$\endgroup\$
    – Voltage Spike
    Oct 4, 2018 at 4:23

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