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Replacing resistors in small MCU circuit

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neazoi

neazoi

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Hi in this circuit **broken link removed** I wonder if:

1. Can I replace R1 with 1K? Will it matter in the RS-232 port signals?
2. Can I replace R6 with 1k? It is just part of an AF filter after all which is not steep.
3. Can I replace R4 and R5 with 6.8K instead? These are just pullups I think
4. Can I replace T2 with another type, say 2n2907? Or does it matter in the RS-232 signals? (eg, low vce sat etc)
 

Hi,

Please upload the gif here in your post.
Use the "insert image" button.

Klaus
 

Here it is
schematic_simple.gif
 

The use of RS-232 signals without a negative bus reference, in spite of working many times, it is not the correct way to do that, you are not sure that what worked well interfacing this board with one PC, will always work well in another one.
 

Hi,

I assume the negative voltage is generated by a square wave on pin 4.

If you don't have 1.5k ohms resustirs, but 1.0k resistors,
then connect two 1.0k in parallel to get 0.5k
And one 1.0k in series to get 1.5k

Klaus
 
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    neazoi

    neazoi

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KlausST said:
Hi,

I assume the negative voltage is generated by a square wave on pin 4.

If you don't have 1.5k ohms resustirs, but 1.0k resistors,
then connect two 1.0k in parallel to get 0.5k
And one 1.0k in series to get 1.5k

Klaus
Click to expand...

No if 1.5k is critical for the operation I won't change it, it doesn't worth it.
How about the other points?
 

1. Given that you are already breaking the RS232 specification, changing R1 to 1K won't make much difference. It is there to allow enough current to flow that the RX input gets pulled low by the driving signal but not so much current that the RS232 TX line is overloaded. In other words a compromise, a real RS232 interface wouldn't draw as much current.

2. R6 drops the volume and isolates the digital square wave output so the pin isn't overloaded. I wouldn't go much lower than 1.5K to be honest or SCART output will be too high and distorted. It already looks to be much higher than normal.

3. Yes, the values do not appear to be critical but bear in mind you will sink almost 1mA down the pull-ups when it only needs to be microamps. You could probably increase R4 & R5 to reduce consumption will no bad effects.

4. The transistor isn't critical at all but do not use a darlington type, it needs the low Vbe to work effectively.

The other components ARE critical or you will get the wrong video waveform produced.

Brian.
 
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    neazoi

    neazoi

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betwixt said:
1. Given that you are already breaking the RS232 specification, changing R1 to 1K won't make much difference. It is there to allow enough current to flow that the RX input gets pulled low by the driving signal but not so much current that the RS232 TX line is overloaded. In other words a compromise, a real RS232 interface wouldn't draw as much current.

2. R6 drops the volume and isolates the digital square wave output so the pin isn't overloaded. I wouldn't go much lower than 1.5K to be honest or SCART output will be too high and distorted. It already looks to be much higher than normal.

3. Yes, the values do not appear to be critical but bear in mind you will sink almost 1mA down the pull-ups when it only needs to be microamps. You could probably increase R4 & R5 to reduce consumption will no bad effects.

4. The transistor isn't critical at all but do not use a darlington type, it needs the low Vbe to work effectively.

The other components ARE critical or you will get the wrong video waveform produced.

Brian.
Click to expand...

1. Thanks I will do this 1k and try it.
2. I have even tried it without any filter in the past just direct pin output to the scart sound in. It worked but the sound was loud as you said and it was more of a square wave. The problem was that with this mod, it caused interference to the video signal. I am not sure why. So I left it as it is. I did a simulation and a 1k won't change many things.
3. I want to use the less values of resistors (KIT design) that is why I mentioned 6.8k because there is already one and it's not far away to the 10k.
4. Will a 2n2907 be ok? I have lots of them.

Yes the video resistors are critical I know that, they define the color and intensity.
 

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