Small NPN 3904 relay style circuit

Click For Summary

Discussion Overview

The discussion revolves around troubleshooting a small circuit involving an NPN3904 transistor used to control a mechanical coin meter. Participants explore the circuit's design, power supply configurations, and potential issues affecting its functionality.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant describes their circuit setup, noting issues with voltage at the emitter of the NPN3904 transistor and seeking explanations for the malfunction.
  • Another participant inquires about the nature of the coin meter, asking if it is a two-terminal mechanical device or a three-terminal electronic counter.
  • A participant confirms that the coin meter is a mechanical device with two leads, which only activates when power is supplied and then removed.
  • Concerns are raised about the power supply configuration, with questions about the necessity of having separate 5V and 6V supplies and the implications for circuit design.
  • Suggestions are made regarding the need for a diode across the coil if the coin meter is inductive, and the base resistor value is discussed as being flexible within a certain range.
  • A participant expresses uncertainty about the exact type of coin meter they have, indicating they are away from home and unable to confirm details.
  • Clarification is provided that the participant prefers to maintain separate power sources to avoid damaging their iPac device used in a MAME setup.

Areas of Agreement / Disagreement

Participants have not reached a consensus on the exact nature of the coin meter or the optimal circuit design. Multiple competing views regarding the power supply and circuit configuration remain unresolved.

Contextual Notes

Participants have not provided complete details on the power supplies or the full circuit schematic, which may limit the ability to diagnose the issue effectively.

Nevian
Messages
3
Reaction score
0
I am currently trying to make a small circuit work, and more than just trying different things until it does work, I wanted to hopefully get an explanation to why it is not working. I am a novice, I only started dabbling in my free time recently and thought I had a decent if only vague understanding of how a NPN3904 Transistor would work. I have two separate circuits. One is a 5 volt circuit with a button on it, the second is a 6volt circuit with a coin meter on it. The button triggers a keyboard press that will eventually trigger that a coin dropped through my coin door, at this time I would like the NPN3904 turn on and flip the coin meter. For whatever reason, when I measure voltage at the emitter of the transistor there only seems to be 1 volt passing through and there for not enough to trigger the coin meter. I am using a 1k ohm resistor on the base of the transistor. What am I missing?

I am attaching a crude drawing, please explain what I am missing or how I can design this so it will work?

http://i1370.photobucket.com/albums/ag263/nevian2/RANDOM/npn_zpsf9e63613.png~original
 
Last edited by a moderator:
Engineering news on Phys.org
Firstly, welcome to PF. Now the questions;

Is the coin meter a two terminal device? Does it use a relay solenoid driving a mechanical counter?
Or is it a three terminal electronic counter?
 
  • Like
Likes   Reactions: Nevian
It's a mechanical coin meter with only two leads. Once 6v is supplied it clicks over and will only do so again once power is removed and reapplied.
 
Nevian said:
It's a mechanical coin meter with only two leads. ...

doesn't tell us a lot

is it a relay/solenoid ?
at the moment ... where are your 5V and 6V rails being supplied from ?
does the 5V side really need to be 5V or can it also be 6V ?

also a big part of the problem is you haven't shown complete power supplies
I will redraw the circuit for you

Dave
 
OK a pair of circuits one for all 6V supply and one for 5V and 6V supplies
take your pick

Till you respond, I will assume the coin thing is a relay/solenoid type of thing and is denoted by the coil and the relay switch in my diagram
as such, because it's an inductive load, you will need the 1N4007 (or similar) diode across the coil as shown

RelaySw1.GIF


The base resistor value isn't too critical somewhere around 2k2 - 5k6 will be fine

cheers
Dave
 
I am not certain this is the coin meter I have, as I am not at home, I am at work, but it is similar to this.
http://www.twistedquarter.com/index...ducts_id=848&zenid=j84hbcueaagphq8kvdlaglljt4

Sorry about the late response, I work nights on the Pacific Coast. Thanks for the help.

As for either ckt being supplied from the 6v, I have two separate power sources and I would like to keep them that way, I do not want to accidentally fry the iPac that I am using in my MAME.
 
Last edited:

Similar threads

Replies
11
Views
3K
Replies
12
Views
3K
Replies
12
Views
3K
  • · Replies 3 ·
Replies
3
Views
2K
Replies
12
Views
4K
  • · Replies 2 ·
Replies
2
Views
2K
  • · Replies 5 ·
Replies
5
Views
2K
  • · Replies 1 ·
Replies
1
Views
3K
  • · Replies 13 ·
Replies
13
Views
4K
  • · Replies 5 ·
Replies
5
Views
4K