Will this circuit work

[lew247]

It's literally an rs232 (ttl level) serial link
You have one at the remote end, and one on the other end and its like having the serial ports connected with wires, but its wireless

**broken link removed**

User manual here **broken link removed**

 

[MikeMl]

I would take the claimed 1000m range with Two grains of salt...

 

[lew247]

Agreed, it will work over quite a long distance and IF you use the external antenna option (it has a socket on board) and use the 100mW output you should get the 1Km range easily enough, but the transfer speed will be only 1200 baud

 

[MikeMl]

... and you live in the country away from a large metro area, and away from any Ham radio operator who happens to use frequencies near 433Mhz. I have played with these simple 433MHz links which utilize a super-regenerative receiver. The simplistic receiver can be "blocked" by any stronger RF carrier signal within the receiver's passband. Unfortunately, the simplistic super-regenerative receiver is as broad as a barn door. I can prevent reception by keying my 50W Ham transmitter anywhere within ~+- 10MHz of 433MHz...

I switched to using the 318MHz ISM band for this reason...

 

[granddad]

I have the 433Mhz Aurel RTX working at ~30m distance , but protocol needs error checking, and re-TXing if required ( I use a sum check and message number system ) 200m range would great. Not sure if 318Mhz is legal in UK

 

[Nigel Goodwin]

I'm confused. Do you talk to the HC-12 using it as a PnP USB device, or is the communication to its Rx/Tx pins just RS232 serial?

Just 5V RS232 serial.

 

[spec]

I've been told this circuit will not work and I need to use an opto isolator as there is no ground for the transistor
The chip is a pic32
I say it will work fine it's a simple switch, the transistor conducts and the ground is the other end of the module, in this case a HC-12

Hi lew,

This is the best bet for switching shield power lines from a controller. It provides isolation between the controller positive supply line and the shield supply positive line so that the shield positive line can be any voltage you like within reason. Of course, the circuit can be scaled for higher voltages/currents if required. The circuit also gives a very low voltage drop between the shield power supply + line and the shield + supply line.

spec

 

[MikeMl]

...This is the best bet for switching shield power lines from a controller...

Similar to what I posted back in #4. Lew didn't like it because it "has too many parts..."

 

[JimB]

Not sure if 318Mhz is legal in UK

No, it is not.

JimB

 

[spec]

Similar to what I posted back in #4. Lew didn't like it because it "has too many parts..."

Apologies Mike,
I obviously hadn't read all the previous posts fully. My circuit is similar to yours, but yours is better.
spec

 

[throbscottle]

And with a dual transistor it's just 3 parts. I had a quick look at the RS website: BC817UPN, SMBTA06UPN and MMDT4413 are all adequately rated for the job, though the SMBTA06UPN has the lowest Vce(sat) of the three.

 

[granddad]

Permission to disagree with post #67, There is a possibility for the circuit to turn itself on.. a floating base ( during control device start up ) Q2 may start to conduct , voltage may be present on inter- connections (TX) anything could happen ! Much prefer control going low to turn on...

 

[spec]

Permission to disagree with post #67, There is a possibility for the circuit to turn itself on.. a floating base ( during control device start up ) Q2 may start to conduct , voltage may be present on inter- connections (TX) anything could happen ! Much prefer control going low to turn on...

Hi grandad,

Q2 will not tend to turn on if its input is floating: it is not a FET with a high input impedance. For belt and braces you could put 10K Ohm resistors between the base and emitters of both transistors, but it would not be necessary.

There is more chance of the output signal being low on turn on than being high. I suggest the a positive turn on is better. It also allows a simple assertive hold off at turn on by adding a capacitor between the base and emitter of Q2.

If you had voltages on the interconnections of circa 400mV which would be needed to turn Q2 on there would be something wrong somewhere. If that were the case you could multiply the turn on threshold of Q2 by a corresponding resistor between the base and emitter of Q2.

Both approaches have their pros and cons but, on balance, I prefer the positive on approach.

However, if you really must have a not turn on function that would be simple to implement too.

spec

 

[Nigel Goodwin]

Permission to disagree with post #67, There is a possibility for the circuit to turn itself on.. a floating base ( during control device start up ) Q2 may start to conduct , voltage may be present on inter- connections (TX) anything could happen ! Much prefer control going low to turn on...

That's not how PIC's start up - pins always start as inputs, you configure them as you want. In this case you would set the output pin LOW, and then set it as an output - so there's no possibility of it inadvertently going high.

However, I would like to see resistors between base and emitter of the transistors, and my previously mentioned tutorial shows one on the top transistor (which I considered the most important of the two).

 

[spec]

Here is a NOT switch, based on MikeMI's circuit of post #4, with a threshold of: (controller positive supply line /2) - 500mV:

​

 

[spec]

Here is a power switch, also based on MikeMI's design, which has an on threshold of 2V:

 

[spec]

This single component interface will switch a large current with the following provisos:
(1) The MCU and shield are powered from the same supply lines, both positive and 0V.
(2) A NOT on function can be tolerated.
(3) The power supply has a sufficiently high voltage to turn the PMOSFET on enough to provide adequate current to power the shield.

​

NOTES
(1) The MOSFET switch can be driven by both the normal current and high current MCU outputs.
(2) It would be wise (IWBW) to put a 22R resistor directly on the gate of the PMOSFET to act as a gate stopper to prevent parasitic oscillation.
(3) IWBW to put a 10K resistor between the non MOSFET gate end of the 22R resistor above and the MCU + supply line.
(4) IWBW to connect a 100nF, or greater, ceramic capacitor between the PMOSFET source and MCU 0V supply line.
(5) IWBW to connect a 100nF, or greater, ceramic capacitor between the PMOSFET drain and MCU 0V supply line.

 

[spec]

Here is an MCU/shield power on/off circuit with the following characteristics:
(1) Only two components
(2) Positive turn-on
(3) Allows MCU and shield power + supply lines to be separate.
(4) No loading on MCU output
(5) Functions with an MCU + supply line down to 1.8V
(6) Functions with a shield + supply line down to 1.8V
(7) Functions with an MCU + supply line up to 12V (20V with minor mods)
(8) Functions with a shield power + supply line up to 12V (20V with minor mods)
(9) Supplies up to 1A shield current
(10) Low voltage drop between shield + power supply line and shield + supply line (135mV worst case @ 1A)
(11) Very low cost

​

DATA SHEETS & SOURCES
(1) NXP PMCPB5530X NMOSFET & PMOSFET
**broken link removed**
https://uk.farnell.com/nxp/pmcpb553...id|77666340848|kword|pmcpb5530x|match|p|plid|

 

[granddad]

pins always start as inputs,

Taken...( My post #54) In my book an input tied to a transistor base is ' floating ' it has no driver.... , ( given no bias resistors ) it is just as easy to set an IO pin, LAT =1 and then TRIS as an output , with no opportunity of losing control, OP wanted minimal components ?

 

[granddad]

(1) The MCU and shield are powered from the same supply lines, both positive and 0V.

Don't think they were ...

A NOT on function can be tolerated.

?