Hello,
I am building an electronic quiz board for our local County Park for junior rangers. I have built a few previously that work on the basic concept and are easy to wire. Unfortunately I am being asked now to incorporate a "wrong answer" indicator on the board as well. A Green LED lights up if they are right and if they are wrong, a red led lights up. Needless to say this is a little more complicated using simple contacts and jumpers. The board has two magnetic pens tethered using wires. One pen is connected to 5v+ and the other pen is connected to 5v-. When the pen touches the two pads that are connected on the back, the answer is correct, the circuit is complete, and the green LED lights. When the pen touches pads that are not connected nothing happens. Instead of nothing happening, they want a red LED to light. . I am posting an example of one of the existing quiz boards. Can someone lend some insight on the easiest way to make this happen outside of using ICs? I was hoping this could be done using just diodes and relays, but I have exhausted my knowledge on the subject.

Thanks,
Jimmie

 

You can see the pens attached to the board at the bottom of the JPEG.

 

hi Jimmie,
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hi Jimmie,
Your jpg image will not open, it reports an error.
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Sorry, but it seems to be opening okay for me. If you look closely you will see the little metal bolt heads at each square that the magnetic pen attaches to.

 

What are "the connections on the back"? Just wires? Let's clean up the description a bit.

When you say 5v+ and 5v-, I assume you have only one 5V power supply, and these are the + and - terminals, or in a circuit sense, V+ and GND (ground).

And the complete circuit is +5V > wire to left side pen > left side contact > wire > contact under the correct box > wire to LED > GND. Or some other rearrangement of this series circuit.

Is this correct so far? And, what are "magnetic" pens?

ak

 

You simply put in identical circuits to the ones already there and replace the green light with a red one. Touch the wrong one and the red light comes on. Touch the right one and a green light comes on. An LED for every possible choice, some green the rest red.
What is keeping you from doing this?

 

What are "the connections on the back"? Just wires? Let's clean up the description a bit.

When you say 5v+ and 5v-, I assume you have only one 5V power supply, and these are the + and - terminals, or in a circuit sense, V+ and GND (ground).

And the complete circuit is +5V > wire to left side pen > left side contact > wire > contact under the correct box > wire to LED > GND. Or some other rearrangement of this series circuit.

Is this correct so far? And, what are "magnetic" pens?

ak

That is exactly correct AK, sorry for the sloppy description. The magnetic pens are just simple magnets attached to short dow rods. The dow rods are hollow with a wire ran through them that are soldered to the back of the magnet. When the magnet pens (one on each side of the board hooked to + and gnd respectively) comes in contact with the bolts it completes the circuit. See attached pic for the best way I could represent part of the board.

You simply put in identical circuits to the ones already there and replace the green light with a red one. Touch the wrong one and the red light comes on. Touch the right one and a green light comes on. An LED for every possible choice, some green the rest red.
What is keeping you from doing this?

I am trying to use (1) red and (1) green LED for the board. I had not considered using multiple LEDs.

 

Is there a way to do this with just relays?

 

How is it now connected to only light for the correct selection?
You only show one connection in you diagram.

 

How is it now connected to only light for the correct selection?
You only show one connection in you diagram.

Sorry guys, I'm trying to explain this best I can. Have a look at the picture I posted in post #1 of one of the old boards we currently have. It uses names on the left and you have to find the matching picture on the right. They are matched behind the board via a copper wire. A magnetic pen is used to touch a metal tab at both the name side and the picture side. When you select the picture that matches the name, the green LED lights. If you select wrong, nothing happens. Here is another crude sketch of how it works attached. I am basically substituting a simple toggle or momentary switch with the magnetic pen.

 

I am beginning to think there may not be a way to do this without something more than relays.

 

Okay, I see how it works now which is a very simple design. Just had a little brain freeze.
I've given it some thought but so far, I can't think of a simple way to light the light with a wrong selection.
The problem is, it's simple to detect one right selection, but not so simple to detect umpteen wrong selections.
It you dedicated one probe as only for selecting the question, and the other dedicated for only the answer, then that might simplify the problem.
Would that be acceptable?
Edit:
Below is my take on a way to do it with one relay, diodes, and dedicated probes.
It requires one 1N4148 diode (or similar) per answer line (two lines shown).
I didn't show a resistor in series with the LEDs which are needed.
Note that a diode is also required across the relay coil, as shown, to suppress inductive transients.

When the Ans probe and the Select probe are connected to the same wire (correct answer) then the relay is energized and only the Green LED lights.
If The Ans probe and the Select probe are connected to different wires (wrong answer) then the relay is not energized and the Red LED is activated through the diode from the incorrectly selected answer line.

Edit 2--Simplified schematic:

 

crutschow,
Spot on, thank you. I believe thats it exactly. That should work perfectly!

 

See my simplified schematic in post #12.

Edit: Below is the circuit with a P-MOSFET substituted for the relay if you want to go all solid-state (more reliable and lower power).
The P-MOSFET can be just about any logic-level type with a voltage rating of 15V or more and an ON resistance of a few ohms or less.
Note that the MOSFET source is connected to the diodes and the drain to the LED.

For the wrong connection the Red LED is lit through the diode and the MOSFET, since the MOSFET gate is grounded through the resistor, giving a negative Vgs (positive Vsg) nearly equal to the supply voltage which turns it ON.
For the correct connection the MOSFET is turned OFF since the gate-source voltage is now zero, so only the Green LED lights.

 

Hello,

@crutschow , I assume we still need current limiting resistors for the leds.

Bertus

 

Hello,

@crutschow , I assume we still need current limiting resistors for the leds.

Bertus

Of course.
I neglected to restate that.

 

crutschow,
Spot on, thank you. I believe thats it exactly. That should work perfectly!

If you want to minimize part count (eliminate a bunch of relays), you can use two keyboard encoder chips (one of 16 selected keys yields a 4-bit output plus a strobe to indicate a key is pressed).

Run the output of each selection to a 4-bit magnetude comparitor (7485). Run the strobe of the keyboard encoder & the output of the through some glue logic to keep the LEDs off when no guess is made and appropriate Red or green when comparator output shows to keyboard decoders are equal. Should take a total of four chips on a strip board.

If you are in the US, I would be happy to send some old keyboard decoders your way. HD0165 from Harris, a 7485 and a handful of and/not/Or gates to get you through this.

4-bit comparitors would be CD4063 or a 7485

The only issue is that this method requires a monostable to turn power on for a few minutes - other wise batteries will be quickly drained. If you plan to use an AC adapter (cell phone charger, then no problem. (The relay-based system does not consume power unless the jumpers are connected to make a guess - correct me if I'm wrong @crutschow


Here is the logic of the harris chip.

 

Thanks a ton guys! My board will have 16 questions and 16 answers so I modified the schematic a little to reflect that. My last question would be if I can get away with one single diode on the "answer" side rather than 16 individual diodes? I am attaching both ways so you can see what I am asking.

Thanks,.
Jimmie

 

Thanks a ton guys! My board will have 16 questions and 16 answers so I modified the schematic a little to reflect that. My last question would be if I can get away with one single diode on the "answer" side rather than 16 individual diodes? I am attaching both ways so you can see what I am asking.

Thanks,.
Jimmie

I assumed that you need a diode on each of the 15 wrong answers otherwise there will be issues as you multiplex the other 15 relays. Let me think about it for a minute.

 

Thanks a ton guys! My board will have 16 questions and 16 answers so I modified the schematic a little to reflect that. My last question would be if I can get away with one single diode on the "answer" side rather than 16 individual diodes? I am attaching both ways so you can see what I am asking.

You only posted one way.
But you need one diode on each line to isolate the lines.