circuit fade

[camerart]

Hi,

I have built an interface between computer and radio, for sending Morse. See attachment.

I have substituted: C1 2 X 1uF electrolytic polarised caps connected -++-. Q1 BC 548 PNP. D1,D2 1N4148 T1 Maplins LT44 (HX82D)

It works, but after a while seems to fade. I'm not sure if it is affecting the soundcard, or if my circuit is getting hot.

Can anyone tell me where I've gone wrong, and how to improve it.

Cheers, Camerart.

 

[MrAl]

Hi,

How fast do you have to switch the relay here?

 

[picbits]

What power supply are you using ?

 

[camerart]

Hi,

How fast do you have to switch the relay here?

I'm guessing, about 100-200 max/minute. I can adjust it, and it works fine. I have an led switched on the other side of the relay.

 

[camerart]

What power supply are you using ?

For testing, I have a large 12v battery, then i will use a smaller one later.

I am wondering about the 2 X capacitors in series. I am just going to get a proper non-polarised one, in case this is the problem.

I found suggestions that 2 x polarised caps can be used for 1 x non, but suspect this.

 

[MrAl]

Hi,

Do you have to use a relay or can you just use another transistor or just one transistor? Relays that have to switch fast dont last as long. Better to use a transistor.

 

[camerart]

Hi,

Do you have to use a relay or can you just use another transistor or just one transistor? Relays that have to switch fast dont last as long. Better to use a transistor.

I simply followed the drawing, once I get it working I might change it.

I have loads of old relays, and can hear it switching.

I am trying to find out what is fading, would the relay fade?

 

[4pyros]

It works, but after a while seems to fade.

What exactly is fading?

 

[camerart]

What exactly is fading?

That's a good question. One or more of the components, I assume? or even the computer soundcard.

 

[4pyros]

What do you meen by fading? What is happening? Whats wrong? Does it slow down?

 

[camerart]

What do you meen by fading? What is happening? Whats wrong? Does it slow down?

When running correctly, it clicks brightly, then after about 5 mins, gets weaker till the relay doesn't click.

 

[picbits]

I'm sure I posted this question before but when it's "fading", what is the battery voltage ? It may start off at 12v but it could be you have a weak or discharged battery.

 

[camerart]

I'm sure I posted this question before but when it's "fading", what is the battery voltage ? It may start off at 12v but it could be you have a weak or discharged battery.

I use 12v Wheelchair batteries, and they are staying at 12V.

The input to the interface is a miniature transformer Maplins LT44 side one, 2 wires 1000 ohm side two, 3 wires 100 ohm ?? each side. I didn't exactly know which way round to put it, and guessed from earphone 1 tap to centre tap (100 ohm) then towards the transistor 1000 ohm. I'm just thinking that this might be the wrong way round. I'm not sure which way round matching transformer should be. If this was wrong would it fade my computer output?

 

[throbscottle]

I'm wondering if the capacitor is building up a charge as it's used. Maybe try a resistor connected across it to pull the voltage down between keystrokes.

(edit) Actually, re-reading your original post, you say you've substituted C1 with two 1uF caps connected -++- ? Unless you've made a typo, that means one of them is connected in reverse. Not good.

 

[4pyros]

The input to the interface is a miniature transformer Maplins LT44 side one, 2 wires 1000 ohm side two, 3 wires 100 ohm ?? each side. I didn't exactly know which way round to put it, and guessed from earphone 1 tap to centre tap (100 ohm) then towards the transistor 1000 ohm. I'm just thinking that this might be the wrong way round. I'm not sure which way round matching transformer should be. If this was wrong would it fade my computer output?

No thats right.

Actually, re-reading your original post, you say you've substituted C1 with two 1uF caps connected -++- ? Unless you've made a typo, that means one of them is connected in reverse. Not good.

Remove the reversed cap and try again.

 

[MrAl]

Hi,

Really though a relay is not needed here a transistor or two would be better. No clicking, no wearout, even if the circuit works as it is getting rid of the relay would be a good idea.

 

[camerart]

No thats right.

Remove the reversed cap and try again.

Throbscottle and 4pyros,

I have connected 2 x 1uF polarised caps, as the circuit asks for a 1uF non-polarised, and I have read that this is a way of doing it???

 

[camerart]

Hi,

Really though a relay is not needed here a transistor or two would be better. No clicking, no wearout, even if the circuit works as it is getting rid of the relay would be a good idea.

Once I get it going properly, I will do that.

 

[Mr RB]

Throbscottle and 4pyros,

I have connected 2 x 1uF polarised caps, as the circuit asks for a 1uF non-polarised, and I have read that this is a way of doing it???

Yes that is an acceptable way to make a non-polarised cap from twp polarised caps. However the circuit does not need a non-polarised cap! Just use a normal 1uF cap with it's - lead to ground.

You are only half wave rectifying the transformer signal, so you are getting all the power transferred as DC. This will be causing issues over time with the sound card output as it is capacitor coupled.

Replace the diode with a 4 diode "bridge rectifier" so that balanced AC is used from the soundcard->transformer->transistor base. Also you need a resistor across the transistor base (base to gnd), try about 470 ohms to 2.2k. That will give a more defined ON/OFF point and give better relay turnoff.

 

[camerart]

Yes that is an acceptable way to make a non-polarised cap from twp polarised caps. However the circuit does not need a non-polarised cap! Just use a normal 1uF cap with it's - lead to ground.

You are only half wave rectifying the transformer signal, so you are getting all the power transferred as DC. This will be causing issues over time with the sound card output as it is capacitor coupled.

Replace the diode with a 4 diode "bridge rectifier" so that balanced AC is used from the soundcard->transformer->transistor base. Also you need a resistor across the transistor base (base to gnd), try about 470 ohms to 2.2k. That will give a more defined ON/OFF point and give better relay turnoff.

I must admit, I am a bit worried about the soundcard being the bit that's fading.

Your suggestions seem interesting, I will try them, including changing the relay to an output Transistor instead of the relay as suggested by a couple of people. Can someone let me know how to do it, or perhaps use a different transistor that will act as a relay.

I'll report back what happens.

Thanks