Mystery Box

Suppose you had a mystery box with 6 buttons on the top and a

4-conductor cable coming out the back. You know that the other end of the cable is connected to an embedded microcontroller, but you don't know what's inside the mystery box itself (and you can't open the box).

Using an oscilloscope, you observe that the 4 wires in the cable have the following voltages when the system is powered on:

Wire 1 has 5VDC relative to Wire 2. Wire 2 appears to be a ground. Wire 3 has ~9VDC relative to Wire 2. Wire 4 has 5VDC relative to Wire 2.

You also observe that the voltages on wires 1 and 4 change when you press the buttons:

When no buttons are pressed, wires 1 and 4 have 5V as noted previously. When button 1 is pressed, wire 1 drops to 4V. When button 2 is pressed, wire 1 drops to 3V. When button 3 is pressed, wire 1 drops to 0V. When button 4 is pressed, wire 4 drops to 4V. When button 5 is pressed, wire 4 drops to 3V. When button 6 is pressed, wire 4 drops to 0V.

What's your best guess regarding the circuit inside the box?

Reply to
Fred Fuque
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Is this your lab homework or something?

Reply to
wolahr

It's useless or broken.

Cheers

Reply to
Varactor

Six switches and some resistors. This encoding technique has been used in automotive applications to minimise the number of connections that need to cross slip rings or flexible interconnects when getting signals from the steering wheel of a vehicle. The microcontroller probably had a couple of spare adc channels available. It avoids EMC issues associated with digital multiplexing on unshielded wires and is very cheap.

John

Reply to
jrwalliker

OK, but wired how? A set of SPST momentary switches, each wired between +5V and GND through a resistor?

In this case, it's the control box for a radar detector:

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I want to replace the original control module with switches that look more "factory" but I'm having trouble figuring out how to wire it.

Thanks for your help!

Reply to
Fred

3 ---+-------------------------+ | | [R] [R] | | +---+----+----+---Wire1 +---+----+---+---Wire4 | | | | | | | | [R][PB1][PB2][PB3] [R][PB4][PB5][PB6] | | | | | | | | | [R] [R] | | [R] [R] | | | | | | | | | 2 ---+---+----+----+-----------+---+----+---+ 6 N/O pushbuttons and 8 resistors.

Ed

Reply to
ehsjr

Thanks, Ed.

One thing I forgot to mention: there is 5V present on 1 and 4 even when the control module is unplugged from the main unit (i.e. the embedded micro with the two ADC channels is supplying 5V on these lines even though they are apparently outputs from the control module to the micro). Is the above circuit consistent with this observation?

Thanks again!

Reply to
Fred

I've seem it used in a VCR too, some even had 2.5mm sockets on the back you could plug a resistor array based remote control into.

but the remote used only 2 wires.

You don't even need a full ADC, a RC multivibrator is enough,

Reply to
Jasen Betts

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to decode these use 'view source' in the options menu,

3 ---+ | 10K | 1 ---+-----+--------+--------+ | | | | 12K 22K 8.6K | | | | | | O | O | O | | | 1 | 2 | 3 | O | O | O | | | | | 2 ---+-----+--------+--------+ 3 ---+ | 10K | 4 ---+-----+--------+--------+ | | | | 12K 22K 8.6K | | | | | | O | O | O | | | 4 | 5 | 6 | O | O | O | | | | | 2 ---+-----+--------+--------+

where K is not nececarrily kilohms but is a constant.

measure resistance from 3 to 2 while buttons 3 and 6 are depressed. divide that by 5 to find 'K'.

Reply to
Jasen Betts

possibly.

disconnect it and then measure it again,

Reply to
Jasen Betts

OK, I did that, and verified that there is 5V present on 1 and 4 even when the control module is unplugged. I also noticed something else: the voltage on 3 also varies.

Let me backtrack a bit.

The control module has a total of 6 buttons: an on/off button on top, and 5 buttons on the front.

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It's not apparent from the photograph, but the 5 buttons on the front are back-lit with little blue lights. The "SENS," "MRK," "VOL," and "BRT" buttons are lit as a group, and the "MUTE" button is lit independently of the other 4. The main unit makes the Mute button flash when radar is detected.

While I was poking around with the 'scope I discovered that when the main unit is "off" (it's not completely powered off since it is still supplying 5V to wires 1 and 4) there is 0V present on wire 3. When the unit is turned "on" and the 4 corner buttons are lit, wire 3 has ~4.5V, and when all 5 buttons are lit ~9V is present on wire 3.

All of this seems to suggest that the voltage on wire 3 is only used to light the buttons and is not the source of the voltage present on 1 and 4. Somehow the 5V on 1 and 4 serves as both a reference voltage for the switches and the input to the ADC.

Reply to
Fred

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That changes almost nothing:

----------------- | Main Unit | | Gnd +5 +5 +9* | | | | | | | | | [R] [R] | | |__|___|___|___|__| 2| 1| 4| |3 | | | +----------------------+------> to corner lights | | | | | | +---------------------+ [Zd] | | | | | | | +-----> to remaining light | | | | +---+----+----+ +---+----+---+ | | | | | | | | | | [R][PB1][PB2][PB3] [R][PB4][PB5][PB6] | | | | | | | | | | | [R] [R] | | [R] [R] | | | | | | | | | | +---+---+----+----+-----------+---+----+---+

The +9 in the diagram has a * next to it - apparently it can be 0v, +5V or +9V per your revised measurements.

Logically, the circuit is about the same - the change is in the voltage you measured at wire 3, and the location of the resistors in the main unit instead of the control unit.

Ed

Reply to
ehsjr

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that makes sense, eg:

+5V ----+ | R | input---+----- > to buttons (wire2 or 4)

but really you should measure the control unit in isolation,

in particular measure wire-to-wire resistance with the different buttons pressed.

the layout of a circuit to get the same behavior should be fairly obvious.

duplicating the illumination could be a little harder.

Reply to
Jasen Betts

Sorry, I misunderstood what you were asking.

I measured ~3.0 Kohms between 1 and 2 when the "Mute" button is pressed, and open circuit at all other times. Similarly, there is ~1.5 Kohms between 2 and 4 when the "On/Off" button is pressed, and open circuit at all other times. None of the other 4 buttons change any resistance readings.

Does this mean that only the "On/Off" and "Mute" buttons are passive, whereas the other 4 buttons have some active circuitry behind them?

Fortunately I don't care about duplicating the illumination behavior. If my substitute buttons are capable of being illuminated, I'd rather have them light up at the same time as the rest of the instrumentation in the car.

Reply to
Fred

previously you called the buttons 1, 2, 3, 4, 5, and 6

are "on-off" and "mute" buttons that produce a 0V output, or something else?

Reply to
Jasen Betts

Yes.

Reply to
Fred

It occurs to me that maybe I don't need to *replace* the control module; perhaps augmenting it will do. My goal with all of this is to not have an obvious and easily-recognizable radar detector control module sitting out in the open for cops and/or thieves to see. The problem with simply tucking the control module away in the ashtray or the center armrest is that I will need to hit the "Mute" button occasionally while driving, and I'll want to be able to do that without having to open up some cubbyhole or take my eyes off the road.

So I thought of this:

+------+ +-------+ | | | | | C M |1------+------->| | | o o | | | | | n d |2---------+---->| M U | | t u | | | | a n | | r l |3-------------->| i i | | o e | | | | n t | | l |4-------------->| | | | | | | | +------+ | | +-------+ | | | | [R] | (R = 3Kohms) | | | o | | | | o | | | | | +--+

I could continue to use the original control module, but tuck it away inside the armrest so that it's out of sight. I could then tap into two of the wires and add a new momentary pushbutton switch in parallel to provide a readily-accessible Mute button. This new button could easily blend in and look "factory," achieving the desired goal of stealth. Meanwhile, the other buttons are still available, and since I don't need to change the settings very often (and not at all while the car is moving) it's not a problem to have those buttons hidden away.

Any obvious problems with this approach? Would anything blow up if I were to foolishly press both the real Mute button and my new "piggyback" Mute button at the same time?

Reply to
Fred

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