Transistor switching question

[Torben]

Hi all,

I have built a foosball scoreboard which works fine for me. It is based on TTL logic for the counting and LED display, and 555s for the trigger debouncing and win detection. The 555s trigger 3904s to light red 12V/20mA lamps when either player scores, and blue ones when either player wins. Now I want to add outputs which I can plug into the trigger inputs of my SPD-20 (Octapad-like unit with internal sound generators).

The circuit segment in the first attachment shows how I am currently switching the lamps in the bottom half of the image, and how I think I might trigger the SPD-20 in the top half. I tested the inputs on the SPD-20 and I like the sensitivity range it shows using 0.9V pulses, so that's what I set the voltage divider to for that section. (Actually, here's a side question: at the behest of someone I'm not sure I trust too much, there is also a 104 ceramic cap across each 3904's emitter and collector, and a 1n914 diode across each lamp--I think the diode is there because I mentioned the thought of powering small motors to ring bells instead of using the SPD-20. What are the caps doing there? Anything? It does work with them there...).

All lines coming from the BRAIN header are +5V pulses from the 555s.

I suspect the circuit will work (maybe even this weekend), but I wonder whether I couldn't save some parts and share the resistors between analogous 3904s. i.e. the SCORE1 lamp transistor and the SCORE1 SPD-20 trigger transistor would share a resistor, instead of having separate ones. Bad idea? Good idea?

The second attachment shows another idea I had, but this one doesn't seem so safe. It seems like I might get more voltage than I want kicked back into the SPD-20 trigger inputs (I don't want to send 12V up there...they seem pretty hardy, but I don't want to push it). Again: bad idea? Good idea? Any way to make it a good idea, if it is in fact a bad one as shown? Maybe resistors between the SPD plug/lamp junctions?


Thanks for any input!

Torben

 

[ericgibbs]

hi torben,

a 104 ceramic cap across each 3904's emitter and collector, and a 1n914 diode across each lamp

The cap is a bad idea, if it across the collector/emitter, as you say driving LED's the diode is not necessary.

The second circuit with the SPD connected as drawn could damage the inputs to the SPD.

On your first cct, why dont you connect a diode is series with the 0V and emitter pin on the lamp driving transistors?
Forward biassed it would give about +0.7V for the SPD inputs when the lamp is ON. Wouldnt need those other transistors.

Eric

EDIT: error in drawing diodes, Redrawn.

 

[Torben]

hi torben,

a 104 ceramic cap across each 3904's emitter and collector, and a 1n914 diode across each lamp

The cap is a bad idea, if it across the collector/emitter, as you say driving LED's the diode is not necessary.

Actually, they aren't LEDs; they're filament lamps. Not sure if that matters. The ratings are still 12V/20mA though.

The second circuit with the SPD connected as drawn could damage the inputs to the SPD.

Ok, that's what I feared.

On your first cct, why dont you connect a diode is series with the 0V and emitter pin on the lamp driving transistors?
Forward biassed it would give about +0.7V for the SPD inputs when the lamp is ON. Wouldnt need those other transistors.

Eric

I suspect the answer to your question here is "my lack of experience".

(edit: the last paragraph was written before I saw your attachment, and the attachment pretty much answers the question that was in it, so I've removed it.)

It seems this also leaves only 11.3V for the lamps, but I really can't see that being a problem.


Thanks!

Torben

 

[Torben]

Hi again Eric,

Just one thing about the schematic you attached: is it just me or are the diodes backward? It looks to me like they should be the other way 'round; i.e. with the cathodes to ground.

Spice appears to agree with this (simmed it to check my thinking), but then I'm not familiar enough with the program yet to tell whether I'm getting useful simulations (I will breadboard this one in a little while here anyway). With the diode anodes to ground (first attachment), I get a slightly rising voltage just north of 4.6V; with the cathodes to ground (the other attachment), I get ~0.6V; which should be fine for what I need.


Thanks for any clarification,

Torben

 

[ericgibbs]

hi torben,

Yes Torben Ive drew the diodes REVERSED!. Sorry.
Drawing has been corrected!.

Ive been standing too close to the microwave oven again.

As the loads are filament lamps, with a slightly lower voltage, it should extend the lamp life, a little.
If brightness is a problem, you could adjust the +12v to say +12.7v.

Eric

 

[Torben]

hi torben,

Yes Torben Ive drew the diodes REVERSED!. Sorry.
Drawing has been corrected!.

Ive been standing too close to the microwave oven again.

Eric

No apologies needed! Made me use my brain. Anyway, glad to read that I'm not mad; I was worried that there was something very fundamental I was missing.


Thanks again for the help,

Torben

 

[Torben]

hi torben,
As the loads are filament lamps, with a slightly lower voltage, it should extend the lamp life, a little.
If brightness is a problem, you could adjust the +12v to say +12.7v.
Eric

I haven't tried it, but I'm not too worried about the brightness. With the addition of trumpet blasts or tympani rolls through the studio monitors, the lights become for effect only, really. Most of the people I play with are musicians and twig far more quickly with audio cues.


Cheers,

Torben

 

[ericgibbs]

hi torben,
Lets know if it works OK
Eric

 

[Torben]

hi torben,
Lets know if it works OK
Eric

Hi Eric,

It works quite nicely. At first I got the SPD-20 triggering on the falling edge, but swapping the polarity on the trigger input cured that.

I also removed the caps across the transistors and the diodes across the lamps. Still working nicely.


Thanks again for the help!

Torben