I am trying to organise a Bluetooth receiver on board an HO train and powered off the track as shown in the attached diagram.
The aim is to have the Bluetooth receiver active when the track voltage is greater than one volt and switched off when the track voltage is less than one. HO locos seem to start moving at 1.5 to 2v.
Hence the sounds will be active when the loco is stopped at a station as long as the controller is not turned all the way down to zero. When the loco is isolated, to run a different one, then the sounds will go off.
Please do you have any ideas for a circuit that would do this? I would prefer not to have a battery on board if possible.
1 Like
Hi Peter
I don’t think this is as simple as it looks.
First question. Is that diode bridge an existing set up with other things attached. What I am getting at is the ground point (- on the bridge) ground for any other bits or is the system in general using the negative side of the track supply. You indicate ± 0 to 10V so it would look like the bridge is a permanent fixture as part of the whole system and the general ground is bridge negative.
This needs clarifying as there will be a possible ground conflict here.
Where are you referencing the 1V?? The bridge output or track neg.
The bridge output will be 1.2V to 1.4V lower than the input, 2 diode drops. This means you really reference to bridge neg as at iV input the bridge output will be zero (or should be).
I am sure there are some 3.3V regulator ICs used in a few devices which still produce 3.3V down to about 1V or so as well as being 3.3V regulators for voltages above this. There is omit limit to the Max allowable but you will have to do this research. I am not feeling too well at the moment so can’t get motivated, sorry.
Cheers Bob
1 Like
I agree it isn’t simple … I have been mulling it over for days.
The bridge is my addition and is not the general ground … which would be the track.
I forgot about the voltage drop over a diode which I understand to be about 0.3v per germanium diode.
Thanks for your help … get well soon.
Peter
1 Like
Hi Peter
You have the voltage drop over 2 diodes in a bridge rectifier circuit.
Cheers Bob
Have not heard of Germanium for a lot of years. But then again I have not looked either.
1 Like
Hi Peter.
Just how much current are you going to require from that red box in your drawing.
I have a couple of ideas but this current has to come from somewhere and I don’t see the track supply doing it, particularly at the lower voltages. I really think a battery will be required. Say a 18650 rechargeable or one of the flat pack 3.7V rechargeable might have to be considered.
Give me a couple of days and I will publish a simple circuit I think would work OK but a 3.7V source (battery) would be needed.
Cheers Bob
1 Like
Bob,
The power available at the track would be around 15 watts; peaking at 1.25 amps and 12 volts.
Thanks
Peter
1 Like
Hi Peter
That does not tell us what you require out of that red box.
While we are here what is the nature of the signal at the tracks. Straight DC, PWM or whatever. With the idea I have in mind it needs to be DC. PWM would require more thinking.
Cheers Bob
1 Like
Bob,
I don’t know what power the MH-M38 Bluetooth receiver amplifier needs but I will only be driving a single two watt speaker so it wouldn’t be much.
The big problem is finding a step up/step down voltage regulator which will deliver 3.7v or 5.0v to the receiver from an input of 1v to 15v and nothing below 1v. No such regulator appears to exist. However a bit of lateral thinking gave the attached solution…. a resistor on the motor so that the loco starting voltage is increased to 3.5v or whatever is needed. Then a standard Pololu step up/down regulator can be used.
1 Like
Hi Peter
The resistor in series with the motor will reduce the speed across the range. It will not be selective.
Cheers Bob
1 Like
Bob,
That’s correct and no problem. Most model railway locomotives move too fast at maximum voltage. I find that I don’t ever go more than half speed.
Peter
1 Like
Hi Peter
That back boost device would be worth trying. You might have to rethink your 1V minimum and replace it with 2.8V as I think this 2.8V is fairly common for this sort of thing.
I can’t see anywhere any description of what happens below 2.8V. All I can find is a reference to a low voltage lock out which would be fine if one could find out what is required to restore normal operation after being locked out. If it is just a matter of returning to 2.8V input that would be perfect. Just about what you want if you ignore the actual voltage values.
For the price I think this would be worth trying. I like the “soft start” feature.
Don’t forget this ground is separate from all other grounds due to the bridge configuration.
Your idea of forcing the motor start voltage to be larger to get more volts into the bridge at motor start point could work. A bit of experimenting on your part required here.. The down side is wasted power and the resistor could get quite hot. Be careful. If you can afford some wasted power this could be the easy way out.
As long as the track signal is NOT PWM.
Give it a try anyway
Cheers Bob
1 Like
Bob,
Thanks. Very helpful. If the supply is PWM where do the problems occur?
Peter
1 Like
Hi Peter
PWM is a pulsed waveform. Going from 0V to full supply. The pulse width is changed to alter motor speed. The more time the pulse spends “ON” the faster the speed. A DC multimeter will measure the “AVERAGE” DC voltage. Reverse operation is achieved by reversing the connections to the motor or reversing the voltage applied to the motor (which is really the same thing).
If this motor is simply driven by a variable and reversible steady DC voltage you should be fine. But if it is PWM I am not sure how the bridge will behave. At best you will get a series of pulses which will have to be smoothed. With PWM the running motor will modify the pulse seen across it (the motor becomes a generator between pulses) so this is why I don’t know what you will finish up with at the end of the day.
I really don’t know how speed is controlled in a modern train set up. I suppose I could research it but just don’t have the time. Perhaps someone with the experience could chip in and enlighten us.
Cheers Bob
1 Like
Bob,
Thanks. I was going to add a “stay alive” capacitor anyway to cope with momentary breaks in track supply when the loco crosses gaps in the rail. I have ordered the bits from Core Electronics so will do a bit of “suck it and see” next weekend.
Peter
1 Like
