# Simple Circuit Design

• posted

An old friend of mine asked me to help his daughter work out a project. I worked it out rather simply with switches, but that's not what he wants.

Here's the deal. His daughter has a skull that has thirteen different areas. She wants to have thirteen different sheets on the table with a "contact" (thumbtack) on each sheet and a "contact" (thumbtack) on each of the thirteen areas of the skull that correspond to those sheets.

She wants to take a probe from the thumbtack on a sheet and touch it to the contact on the skull. If the person touching the probe from the sheet to the correct contact on the skull, it lights a green light. If they touch it to the wrong contact on the skull it lights a red light.

Given a lot of digital logic or a microprocessor, I can do it easily. Given a boatload of diodes, I can do it easily. I can't do it trivially.

Am I missing something or is this not a trivial problem?

Jim

• posted

Suppose you have a simple voltage divider that creates 100mV to 1300mV in 100mV steps, for the sake of argument. You wire each output to a pair of tacks (one on the skull and one on the sheet)

Add a window comparator to detect if the inputs are within 50mV of each other (green LED) or more different than 50mV (red LED)... (the two inputs will be the two ends of your probe) and a pull down to ground on each input

And another pair of open-collector output comparators to detect < 50mV (open) at either input and shunt (blank) both the LEDs...

You can use LM339/LM324 type parts to build this. The LTC1042 would be ideal, but I'm not sure it's still available.

Best regards, Spehro Pefhany

```--
"it\'s the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com```
• posted

So each of the contacts on the sheet is wired 1:1 with the corresponding contact on the skull, and the probe is a continuity tester? If you make the probe a pushbutton too (with the tack glued to the button) it could turn on a red light if there's a push but no continuity.

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• posted

pull-up resistors. wires directly from the skull pins to the corresponding the sheet pins, pull-up resistors on all the wires

one probe grounded, the other probe designed to give a red light when pulled up and a green light when grounded.

Bye. Jasen

• posted

I may have been totally wrong with my last answer...

how many probes, and how many lights are there meant to be? how should the lights behave in the absense of a circuit? what does "take a probe from" mean?

is there a requirement that the probes are opposite ends of an isolated piece of wire?

conductive paint on the skull could be neat.

Bye. Jasen

• posted

Better use LM3914 and give the common output to signal pin. U have 10 outputs with LM3914. If U want 13 then cascade another. Use in DOT mode only to get 13 outputs. If not required to have separate output then use correct output as Green LED and all others tied up as RED led. Needs no micro no switches. Can work from 3V to 18V supply.

• posted

Okay ... we've assigned sheet contact #7 on the desk and point contact #7 on the skull to the cerebellum, just for sake of discussion ... relative to ground these two points are +700 mV and relative to each other they are zero mV.

But they are ALWAYS within 50 mV of each other; they are hardwired together.

or more different than 50mV (red LED)... (the

How can they be different than 50 mV?

and a pull down to

Now you've totally lost me. If I do a pulldown to ground, I've blown my voltage divider all to hell. And what is an "input"?

Wouldja try again for me please?

Thanks,

Jim

• posted
3914 is an out of production part and difficult to come by for the average person.

Jim

• posted

nope

and is in stock at farnell

martin

• posted

Perhaps I misunderstand the problem.. but I thought the idea was to detect whether the part of the skull was properly identified...

We're in an analog world here. The pulldowns are gonna be orders of magnitude higher resistance.

Vcc o | +---------------------+ .-. | | | | | | | | | | '-' 1300mV | PAGE #13 | SKULL #13 +------------+---o o | .-. 100R | ^ ^ | | Probe A| | Probe B '-' | | o-----------| | | Vcc #12 | | | | .-. ------+ | +-------- 100R | | | | | | | | .-. .-------------------------. .-. '-' 100K| | | | | | | | | | | | |100K /- - - / '-' | COMPARATOR | '-' | | | | #1 o-----------| 100mV | '-------------------------' | .-. === | === 100R | GND | GND | | | '-' | |A- B| < 50mV GREEN LED | | |A- B| > 50mV RED LED === === GND GND

except if(AWouldja try again for me please?

If this isn't what you want, maybe you could 'splain where the interpretation is wrong?

Best regards, Spehro Pefhany

```--
"it\'s the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com```
• posted

on

zero

together.

Maybe it would make more sense if Spehro had said ":two simple voltage dividers - - one on the skull series of contacts and one on the sheet series". The probes are the inputs of the window comparator controlling the red or green LEDs. Its pulldowns to ground would be maybe 1Meg while the dividers would be 10K dividers so that the 1Meg loads would not move the tap voltages much. The blanking comparator would differentiate between the small voltage difference when two correct contacts are probed (due to the imperfections in the dividers) and the zero difference between probe inputs when their pulldowns produce ground level at both probe inputs. (It might be desirable to purposely shift the two dividers 10 or 20 mV rather than relying on random imperfections to produce non-zero probe inputs.)

I saw the circuit diagram that Spehro made, but the columns he set up were not preserved in my display, so it was hard to follow. I was prepared to defer to Spehro's clarification, but his graphic didn't work for me, so here's my attempt to make it more understandable.

Chuck

• posted

Geezzz! Let the kid solve her own problems, no wonder this generation kids are so fat and lazy, daddy's does everything for them.

• posted

If you are happy to split the 'probes' in two parts then there is a simpler way.

Wire together each pair of tacks. From each pair connect 1k to a common point and 1k from there to ground.

One probe is simply a (say) 3k9 resistor to the supply +ve, lets say 6v.

When you connect this probe to a tack that pair of tacks will have 2v on them, the other 12 tack pairs will have 1v.

Now the second probe just has to look for:-

0v - no LEDS 1v - red LED 2v - green LED.

Something like this maybe...

+6v | .-. | | | | '-' | | o-------->|-------. | | | | V V - GREEN - RED | | .---------------------. | | | | | | ___ |/ | ___ |/ --o-|___|-->|---| '--|___|--| |> |>

GND | | -----------------o-----------------'

• posted

on

simpler

I like your idea, but I'm not familiar with ASCII schematic symbols as relates to electronic components. Is there a reference you could steer me to so I can understand your ASCII schematic? I suspect it needs to be in Courier Font, and I think I have my reader set for Times Roman, so I may have to copy the schematic to the clipboard and change font there, if that's possible.

Thanks,

Chuck

• posted

1300mV

#7

to

that's

I tried it, and it works - - just copied your post to Notepad and changed font - - nice.

• posted

You tried the circuit? or just viewing in notepad?

Any fixed point font should do. I should also have credited AACircuit

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