Comments invited: proposed remote volume control

[tcpip]

Dear all,

Am thinking of building a remote volume control at lowest possible cost, using a motorised pot, without having to program any uC myself. After looking in various places, I'm thinking of going the following way:

• The remote handset and receiver will be built around a kit from Reynold's Electronics (details here). I'll probably design a small PCB for the receiver circuit.
• The receiver IC will use only two of its output pins, corresponding to the volume up/down buttons on the handset. Each output pin will drive two transistors (BJT? FET? don't know). These transistors will be "on either side" of the motor winding of the motorised pot, and will switch the current flow through the motor. One pair will pass current one way, the other in the other direction.

I'm planning to use a 9V single-ended supply for the entire thing, including motor. I'll use a 78L05 to drive the receiver circuit, and arrange the NPN transistors such that one pair drives the motor in each direction. The current will flow from collector to emitter of the first transistor, then through the motor, then again through the collector to emitter of the second transistor, thence to ground. For rotation in the opposite direction, I'll use a second pair of transistors. Both pairs will be driven directly by the output pins of the receiver IC.
My questions:

• Anything wrong with my approach? Will this work at all? What should I look out for? I'm a newbie to most things, including uC based circuit design.
• What motorised pot should I use? I can't hear the differences between Alps, Bourns, Clarostat, and other pots, so should I use the Xicon/Alpha device (Mouser part number 316-1020-100K)? I need a linear taper pot, but the documentation on the Mouser site does not tell me what taper it is. Is there any other pot of comparable price from anywhere else? I need a dual-ganged 100K linear pot, if possible. I know some online stores which sell good Alps or Panasonic pots, but they are USD 50.00 and above.
• What transistors should I use to gate the current flow through the motor? I need the device to carry up to 200mA for a few seconds at a time, and create a potential drop of 5-6V across the motor, typically.
• Do I need to build any transistor protection components into the motor control circuit, the way one uses diodes across the coil of a relay?
• Left to myself, I'll use the BD135/137/139 transistors (any of these will do). But the saturation Vce for these devices is 0.5V, which means the drop across two devices in series will be 1V, leaving the remaining 8V to drop across the motor. This may be too much, considering that these motors seem rated for 3-6V. Should I put a series resistor to drop the voltage across the motor to a lower value?
• The Vbe of these transistors appears to be 5V max. If I drive their bases directly from my uC output, they'll get 5V. Should I then put base resistors on the transistors to reduce the Vbe to a lower value?

Sorry about so many questions, but I'm pretty new to all this. It seems doable to me, but then it always seems that way to ignoramii. And I guess it'll be helpful to post a schematic of my proposed circuit.... I'll do it if you want it.

Thanks in advance,
Tarun

[dine1967]

Tarun,
I used the same chip for my preamp remote control. The only thing is that the output of the chip is not momentary( mine was IR -D14 chip). You will have to convert the output using edge trigger to make it momentary. The output is used to drive a relay. For this any small signal transistor would do. Also if you are using relay, its a good idea to use diodes across the relay coils. There are many outputs from the chip and u can add other applications such as mute, source select etc.
I have also used the alpha/xicon 100K motorised pot from mouser. These are linear and are carbon pots. I just bought them to test my whole set up with the intention of replacing them with better quality pot. They are fine but at lower volumes, one can notice the difference in tracking. If u can live with that, go ahead and buy them. Linear pots are not easy to find especially motorised ones.
Hope this helps
Dinesh

[tcpip]

Dinesh,

Thanks.

Originally posted by dine1967
I used the same chip for my preamp remote control. The only thing is that the output of the chip is not momentary( mine was IR -D14 chip).

I was under the impression that both the D14 and D15 had momentary outputs. Anyway, the current page on their site says momentary in quotes, so I presume they are highlighting it this time.

The output is used to drive a relay. For this any small signal transistor would do.

I was planning to use something like a low-power BJT (say BD139?) directly from the output of the D15 chip, and do away with the relay totally. Won't that be good enough? I believe the D15 chip can drive 25mA per pin, and these motor-pots have motors which never draw above 150mA. (All this is "as per specs", but one has to try them out to be certain.) If I assume all this, then any transistor with hfe > 6 and Ic > 200mA should do the job, I should think?

There are many outputs from the chip and u can add other applications ...

Yes, I was tempted. But for the first version, I've decided to keep it limited just to the volume. Someday, I'll add the balance pot too. These two, plus a mute button, are about the only things I need to do from a distance, I've noticed. I rarely change the source from a distance. The balance control becomes handy when badly recorded material plays on my tape deck.

I have also used the alpha/xicon 100K motorised pot from mouser. These are linear and are carbon pots.

Since my last post, I've discovered Brigar Electronics, who have a dual-ganged Alps audio-taper pot for less than USD 6.00. If I can get my hands on some, I'd try that. Am desperately looking for some friend to travel from the US to India in the near future.

Linear pots are not easy to find especially motorised ones.

Yes, and I'd like to use linear ones, because if I cascade a 100K lin volume pot with a 20K balance pot in the scheme that Rod Elliott says, I get a log taper for my volume control. But now that I've found the Alps pots, I don't know how to hook up the balance pot. Will the taper remain substantially a log-taper even now, if I add a balance pot?

Thanks hugely for the inputs.

Tarun