[skyskraper]

hi hi again. sometimes i need to be able to test rca's quickly and thought i could make a little battery powered box to do it with. can any of you guys give me feedback on whether this idea is stupid or if it will work?

i was gonna do it like this:

RCA(only one cable at a time) socket to a spst switch to a LED to the negative of a power source (3v's enough? or 9v and resistor better?) then the positive of the power source to the other rca socket. grounding the rca, on the chassis?

then plug both red's on the rca into the two rca sockets and flick the switch. if the cable is good it should light up right? plugging both white/black plugs in to test that connection next.

i wanted to do one channel at a time with the box so i could easily make something that will fit in my pocket to bring along with me all the time for dj gigs etc....

can u guys give me some feedback on what i plan on doing? im only new at this electronics stuff so please go easy on me

 

[Megaptera]

So you're basically looking to make something that tests an RCA cable for a solid connection between its two end points? If I understand how you want to do it, it should work -- basically using the RCA to complete a circuit that lights and LED, right?

My suggestions:
Use a small battery. Most LEDs only needs about 20-30 mA in order to shine, so you could use a single AAA battery (1.5V); since V/I=R, 1.5V / 0.02A = 75 ohm resistor will get you plenty of light.

I'd also recommend doing the same thing for the ground channel, because as long as you're testing for signal, you might as well do both

That would just be a matter of wiring another LED and resistor in parallel to the signal test circuit. If you want to be extra safe, you could just wire a second battery/circuit in.

Might want to use a plastic case, or at least build it so it's really hard to accidentally short the batteries, unless you want a whole lotta hot.

 

[tkerby]

A better test would look for continuity and shorts. Even better would be to measure resistance to ensure good solder connections but that gets a little more complex.

I suggest two rca sockets on your board. The first has an led, series resistor, toggle switch and battery wired in series and connected between the ground and positive socket connection. The second socket is connected to a normally closed pushbutton.

To test a cable, you connect both ends and throw the toggle switch. THe led should light. You then press the push switch and the led should extinguish ensuring the cable has not been shorted.

You could also make a remote tester with an rca socket and plug with the pushbutton so you can test fixed cable runs

 

[skyskraper]

mega: yup bang on what i want to do. at what point do you suggest i wire in the resistor and second led to test ground?


kerby: so on the negative socket i would just add a push button spst that break the connection when i push it??? with regards to the remote tester, could you explain it a little more, im not quite getting it. a few too many wines too early

 

[skyskraper]

so i had a panel mount 3mm led lieing around so i just wired it up to my battery pack and its not lighting, yet my multimeter says the battery is still giving 1.48v (rechargeable aa

) so i wired up my 9v and gave it a shot and it worked beautifully. so, say i wanted to test both red and black/white rca leads at the same time could i not wire up the second set of sockets (i bought a four panel with a pair of each red and white) and hook up a dpdt switch and an led for each channel (running the circuits in parellel off a 9v). would this be ok?

 

[tkerby]

I think mega is suggesting a check using the ground lead of the rca to complete one circuit and the signal lead to complete another circuit. What I am suggesting is that you complete one circuit only.

The signal goes out the signal lead and is then shorted to the ground by the pushbutton switch and returns down the ground to complete the circuit thus checking both connections at once. Pressing the pushbutton breaks the circuit ensuring that there are no shorts between signal and ground.

To test remotely, wire a pushbutton between signal and ground in an rca plug or socket. Then connect the cable only to the live side of your test box (with the battery and led) and use the remote pushbutton rather than the one in the tester box.

 

[skyskraper]

cool interesting idea. id have never come up with that one myself!!!!

cheers for the help. looks like its off to the store tomorrow to get some push buttons!!!

 

[skyskraper]

ah and a question yet to be answered.... could i double the circuit kerby suggested up and test both left and right cable simultaneously???

oh and sorry another newbie question, when you say two sockets, i already plan on having two sockets, would i just wire a push button to the second socket where one end of the cable is connected and havethe push button (dpdt momentary on im assuming) that makes the signal go to ground?

 

[Megaptera]

My only concern with kerby's suggestion is that if there's a problem with your cable, having a single circuit won't tell you where it is. Testing each channel seperately turns it into more of a troubleshooting tool for very little extra effort. Either one will do what you want; it's just a question of preference.

Either way, you'll need two RCA jacks (sockets are for through-hole components, not cables) to test a single cable at a time. With my suggestion, the jacks will be connected together like this:

.........../- > +battery- ---> signal resistor ---> signal LED --\
RCAin -|--> +battery- ---> ground resistor ---> ground LED -|-> RCAout

The resistor goes between the negative side of the battery and the LED's anode.

 

[skyskraper]

sorry ill remember sockets in the future

im assuming the place to put the switch is as shown below

.........../- > +battery- ---> signal resistor ---> signal LED -spst--\
RCAin -|-> +battery- ---> ground resistor ---> ground LED -spst--|---- > RCAout

ill try it out

just got to go grab another 9v today

and to calculate resistor value hrmmmmm off to remember ohms law again

 

[skyskraper]

ok its working

BUT. before i chop in to this lovely little box i bought for it, i thought i could just drop in a single dpdt in the place of the two spst's. then duplicate the circuit so i can check ALL the connections on a stereo rca cable at once... if i run two more led's off the 9v will that be ok? or will i need to lower my resistor value?

 

[Megaptera]

A DPDT would be fine instead of two SPSTs, unless you can think of any reason why you would want one on and the other off at the same time.

A 9V battery will have plenty of juice for a couple of LEDs. What you want to do is wire them in parallel, not in series, so that they're different circuit paths using the same voltage source. Otherwise, if one path (cable) is broken, they're both going to read as broken. This means that instead of changing the resistor value, you're simply adding a resistor for each LED.

 

[skyskraper]

yah well currently i have each circuit in parallel, just wasnt sure if i doubled the current draw that it might die quickly

cheers. will post pics when im done

 

[Megaptera]

I just looked it up to be sure -- a typical alkaline 9V battery has about 500 milliamp-hours on it. Doing some quick mental math and ignoring quite a few details, if you were to set two LEDs to 25 mA, that'd be 10 hours of diodes emitting light. I'm assuming you probably won't have these on for more than a few minutes at a time, in which case this a single 9V battery should be plenty for you.