# Will my circuit work? (trying to make a who pressed first quiz buzzer)

Discussion in 'General Electronics Discussion' started by Robert Hill, Mar 19, 2015.

1. ### Robert Hill

112
12
Mar 5, 2015
Hi there.
This is the first circuit I've designed.

I'm aiming to make a circuit whereby 2 people have a button and when asked a question the circuit indicates which of them pressed first thus indicating who gets to answer.

Here's the schematic: (I've omitted the means of resetting the 555 timers)

Here's how I hope/think it would work:

At the beggining the output pins are low which means the two LED's light as they are sunk into the 555 timers.
When one individual presses their button (represented by a switch here) two things will happen. Let's say switch 1 is pressed first. First the transistor (T2) attached to the trigger pin of the 555 will be grounded making its input go high and making the LED 1 turn off. Second the base of transistor 3 will have voltage applied. This will open up the collector emitter path of T3. This means that even if the second person then presses their button the current will be shorted to ground via T3. This is because the path to ground through T3 has less resistance than the path to T1 on the second 555 timers trigger pin. This means that the second person who pressed second cannot cause thier LED to turn off. Thus the person who's LED goes off is the person who pressed first.

So, will this circuit work? I'm sure there are more elegant ways to achieve this but i'm just learning so trying to work with what I know.

5,165
1,087
Dec 18, 2013
OK I have never used a 555 timer, so I will have to have a look into how they work. But initially I would say why are you using a 555 timer, do you want it to latch or just stay on while the person is pressing the button? Also you need some LED current limiting resistors and two more base resistors for the transistors T3 and T4 to prevent damage. Move T4 and T3 collectors and connect them to the bases of the relevant transistors. Also some de-bounce capacitors for the switches maybe needed.

5,165
1,087
Dec 18, 2013
Here is my initial version of the 555 timer part. Hope this helps. See if you can workout where to take the hold off from for the other persons button.

chopnhack likes this.
4. ### chopnhack

1,573
354
Apr 28, 2014
Do you intend to tie the reset from the other IC together so that they both reset at the same time?

5,165
1,087
Dec 18, 2013
Hi John
Technically only one would need to be reset at a time, but I am not sure how Robert wants to do it. He may want each person to be able to reset their own LED themselves?
Thanks

6. ### Colin Mitchell

1,417
313
Aug 31, 2014
You can make a much simpler circuit.

7. ### Robert Hill

112
12
Mar 5, 2015
Hi all, thanks for your responses
@ Chophack - Yes I'm planning on the 'quiz master' having a button which will reset all the chips at once after each question (see new schematic below).
@ Colin - Yes, I expect a much simpler circuit does exist. I'm very new to electronics so am just working with what i know so far to design something. I'll look into how others have done it afterwards so I can learn more. Have you any examples of how others have achieved something similar. I'm guessing it will involve logic gates.

@adam. Thanks for your feedback and suggestions. I came to the realization that the best place to block person B's LED going on if Person A presses first would be to ground the reset pin of the 555 timer of person B. Is that what you were thinking?

In terms of debouncing capacitors my research seems to indicate these are usually only needed when dealing with digital chips is that right?

Can I ask what the purpose of C1 and R4-5 are in your schematic?

The parts I ordered came in so i've spent some time building and modifying the circuit i've ended up with the following (as long as I draw the schematic to match what I actually built!) which is even more complicated! This uses four 555 timers and seems to work from my building and testing it.

Can't seem to upload the image at present so here is the link to it on google drive

It works in the following manner. When person A presses their button two 555 ICs are triggered. First the trigger of the 555 sourcing their LED is grounded turning it on, Second it also ground the trigger on their other 555 timer. This makes the output of the second timer high which opens the transistor connected to the reset pin of person B's LED sourcing 555 timer. In this way Person A's light is on whilst person Bs light cannot come on. The same is true in reverse for person B when the chips have been reset. There is a button which grounds the reset pins of all four chips which is held by the quiz master.

Last edited: Mar 22, 2015
chopnhack likes this.
8. ### Colin Mitchell

1,417
313
Aug 31, 2014
Here is the simplest circuit

AnalogKid and Merlin3189 like this.

5,165
1,087
Dec 18, 2013
Hey colin that looks good. But it would be good if we could get his current design working. Once we have done that then yeah we could introduce some different design ideas.

5,165
1,087
Dec 18, 2013
Hey Robert.
Thats a good idea, ground the reset pin. I was thinking of stopping the activation of the trigger transistor like in your origional idea. C1 is the debounce cap it stores a bit of energy so the base of the transistor doesn't wiggle up and down when the switch bounces. Debounce is needed for any signal where if its fluctuation causes an issue. R4 is a pull down resistor and ensures the transistor is off when the base has no signal. R5 is the base current limiting resistor.

11. ### Robert Hill

112
12
Mar 5, 2015
I'm happy to report I now have a fully functioning prototype on my breadboards. I've edited my schematic yet again! This represents my working version:

The main change I made was to make the second non LED powering 555 in each pair ground the reset pins of both of the other persons 555s. This ensures that the person who presses second can't then press their button and end up turning the first persons LED off. I found that both LEDs ended up off with neither person being able to turn theirs on again. I've also added more transistor switches to handle the triggering of the second non LED 555 as before when I used just one transistor to trigger both 555s they seemed to interfere with each other.

I've also managed to create a functioning reset button.

I know it's not the best designed circuit but i'm really pleased with it because i've put a lot of principles which i've learnt into practise both to design it and also to build and modify it. I've learnt a bit about fault finding as well when its not been working as I expected. It's also been good for using the schematic software.

chopnhack likes this.

5,165
1,087
Dec 18, 2013
Well done Robert, nice one. Feels good doesn't it. Now you can think about refining it if your interested.

13. ### Robert Hill

112
12
Mar 5, 2015
Hi again,

Yes it would be good to refine it in order to learn some design principles for circuits. I've been reading a book called 'electronics for dummies' which is good for basics but doesn't tell you how to do more than copy other people's pre existing circuits.

For example building this project I learnt about putting resistors on the base of transistors to stop short circuits.

I think it could be refined by adding the debounce capacitors which you mentioned earlier. Also I could use a Logic gate chip to replace most of the transistors which would allow the circuit to be much more scalable in terms of adding more buttons.

What would you suggest?

5,165
1,087
Dec 18, 2013
Yes you could look at using either transistors as a latch of maybe some invertors (CD4069) or @Colin Mitchell added another form of circuit. There are lots of different ways to do things which is what makes electronics so interesting. Have a look at latches and resettable latches to get some ideas.