Small NPN 3904 relay style circuit

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SUMMARY

The discussion centers on troubleshooting a small circuit involving an NPN 3904 transistor used to control a mechanical coin meter. The user reports measuring only 1 volt at the emitter of the transistor, insufficient to activate the coin meter. Key insights include the necessity of using a 1N4007 diode across the inductive load of the coin meter and the suggestion to adjust the base resistor value to between 2.2k ohms and 5.6k ohms for optimal performance. The user is advised to clarify the power supply configurations for both the 5V and 6V circuits to ensure proper functionality.

PREREQUISITES
  • Understanding of NPN transistor operation, specifically the NPN 3904 model.
  • Knowledge of mechanical coin meters and their electrical requirements.
  • Familiarity with basic circuit design and power supply configurations.
  • Experience with protective components like diodes in inductive circuits.
NEXT STEPS
  • Research the role of diodes in protecting circuits with inductive loads, focusing on the 1N4007 model.
  • Learn about adjusting base resistor values for NPN transistors to optimize switching performance.
  • Explore circuit design principles for integrating multiple power supplies safely.
  • Investigate troubleshooting techniques for measuring voltage and current in transistor circuits.
USEFUL FOR

Electronics enthusiasts, hobbyists working with transistor circuits, and individuals designing coin-operated devices will benefit from this discussion.

Nevian
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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
 
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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?
 
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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.
 
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