Hi all,

The following problem:

I have a model railroad project, where I need to reverse a DC motor (2.4V). I have the same 2.4V and no more than 20mA available as switching current. When on, the motor should be running on reverse. When off, the normal direction. Given all the space in the world, this would be dead easy. However, in this case, even a DIP 16 thing is big. Maybe I can stack two DIP or preferably SMD 8s on top of each other.

I have been looking at quad optocoupler and was quite happy to start with. But now I realise this will switch my thing in one direction on nicely, but there is no way to switch on the other direction, since there is no "off" switches when the switching current is on. Are there optocouplers that act in the opposite direction? I mean, when the led is on, the switch is off. Or maybe there are much better ideas and solutions out there...

Thank you for your time,

/Edmunds

 

What is the available space?

A JFET can conduct current without any signal to its gate (it turns off when there is signal).

 

The space depends on how many components and how big each is. It is an HO scale car, which is already kind of stuffed with decoders, motors, batteries, cables and what not. The passenger cars are like 3.5-4cm (1.5-2") long, so you can imagine. I will go a long way to have it the minimum size possible.

/Edmunds

 

Maybe a tiny relay?

 

How tiny have you seen? Google doesn't help me enough to find anything tinier than usual, small 5VDC thing. Any leads?

Thank you,

/Edmunds

 

How about this:
http://www.rapidonline.com/Electron...s/Microminiature-2A-DPDT-relay/65078/kw/relay
Even better might be an H-Bridge chip like the A3950 from Allegro:
http://www.allegromicro.com/en/Products/Part_Numbers/3950/3950.pdf

 

How much current does the motor draw? Does it ever stall out (which would increase the current a lot)?

 

MPC17510

It wouldn't be the easiest thing in the world to solder, you'd need to etch a circuit board for a TSSOP package (about 1 square centimeter, 12 pins on 2 sides for 24 total).

Not suggested as a first electronics project, but it seems to fit your needs with 2V-15V Operation and 1.2A output/3.8A peak H-Bridge IC. There would be additional components, but not many, the datasheet linked above has typical applications shown on page 10, figures 6 and 7.

Correction: Just read the datasheet and it needs at least 4V for logic supply voltage, motor voltage can be 2V. It uses 3mA for the logic, so a small charge pump IC and some filters could be on the same board to give you 5V logic power from the 2.5V supply.