[General] Any idea if this circuit attached to the MCU pinout is correct?

[wickedweed]

Hi. The attached image is a section of a Car Parking Assist device. Not sure if it is correct but I traced the connections on the board from the MCU pinout to the data out pin of the device using a multimeter.
I am not 100% sure if the transistors are BJTs or JFETs cos they are all SMD devices, but am sure they are NPN and PNP if they are the former.
My guess is that the circuit converts the 5v TTL from the MCU to 12V logic and will be grateful if someone can tell me how this circuit works (or not work cos I may be missing a connection somewhere?)
Thanks!

 

[jayanth.devarayanadurga]

I think you need to pullup the collector of T1.

 

[wickedweed]

I think you need to pullup the collector of T1.

thanks. so the output is inverted?

 

[jayanth.devarayanadurga]

No data is not inverted.

T1 ON = T2 ON = o/p of T2 across Collector is high provided the o/p end is connected to GND internally of 2nd 12V digital device with a pulldown resistor. Connect a resistor from T2 collector to GND and take o/p voltage across it.

 

[wickedweed]

No data is not inverted.

T1 ON = T2 ON = o/p of T2 across Collector is high provided the o/p end is connected to GND internally of 2nd 12V digital device with a pulldown resistor. Connect a resistor from T2 collector to GND and take o/p voltage across it.

Hi. I've checked the connections on the collector of T1 and there is nothing but R2 connected to it. I also used a DMM set to measure up to 2megaohms and this collector junction reads open with ground and open with +12V rail and the +5V rail the MCU is using as well.
The collector of T2 is also not pulled down via a resistor. I measured this junction with ref to ground and it also shows an open connection.
Any idea if this circuit works as is without the pull-ups and downs? Thanks!

 

[Venkadesh_M]

Hi You don need to pull up the circuit you shown can be working fine and if the us gives 1 the output will also 1......

 

[wickedweed]

Hi You don need to pull up the circuit you shown can be working fine and if the us gives 1 the output will also 1......

Thanks!
Any idea what NPN and PNP I can use? What I want to do is to put an RF pair to connect this parking sensor control unit to the display unit.
So I intend to put a RF Tx to the MCU output pin and at the receiver end, the data out from the Rx will go into this circuit and therefore I will have to duplicate this circuit at the other end.
Originally this Data Out on Pad 2 is joint to the Display unit by a wire.

 

[Venkadesh_M]

what is the output, is that a control signal to start and stop display or making a serial transmission ??

If serial transmission why you need high voltage?? for no loss of data ?? then you can just use rs232 or 485 very easily....

 

[jayanth.devarayanadurga]

Yes, Venkadesh is right. No need for Rc for T2. when T2 is ON 12V appear across 190E and current will be 63 mA.

NPN BC547 A/B/C
PNP BC556 / 557

Maybe you need low noise high speed switching transistors.

 

[wickedweed]

Yes, Venkadesh is right. No need for Rc for T2. when T2 is ON 12V appear across 190E and current will be 63 mA.

NPN BC547 A/B/C
PNP BC556 / 557

Maybe you need low noise high speed switching transistors.

Serial transmission. I dont have a logic analyzer so I can't attempt to see what is the protocol used. I guess they used a high voltage to ensure no loss of data cos the cable connecting to the display unit is about 4-5 meters long. I am trying to make a serial RF connection instead of using a 4-5 meters wire cos that will save me the trouble of dismantling my car stuff and leave my vehicle as untouched as far as possible. I'm in fact quite bad at these stuff but find it challenging and fun
Will try using these transistors and see how it goes. Thanks!

 

[Venkadesh_M]

ya that was a interesting stuff and why do you need transistors for making connections with RF transceivers....

 

[wickedweed]

ya that was a interesting stuff and why do you need transistors for making connections with RF transceivers....

I am quite sure that somewhere inside the display unit, this 12V logic is converted back to a 5v TTL and read by another MCU. The problem is this unit has very small SMD parts and it will be quite a pain to solder or make any modifications. So I am trying to do what I can to make it wireless without messing around too much with the display unit's PCB.

 

[jayanth.devarayanadurga]

If you can find the UART pins of the MCU inside then connect RF receiver Dout pin directly to the Rx pin of the MCU UART. This way you don't need the transistor circuit.