Modifying a Blood Pressure Pump to Switch an LED

[Winston Chadwick]

I would like to modify a blood pressure pump to light an LED while the blood pressure test is in progress.

The pump has a motor, two smaller solenoids acting as control valves, and a larger solenoid for the dump valve. When the test starts, the BP pump closes the valves and runs the inflation motor until the max pressure is achieved in the cuff. The control valves slowly release pressure as the user's pulse and pressure range are measured. At the end of the test, the dump valve is opened to fully deflate the cuff.

I would like to add an LED that will turn on when the test starts, and I want this LED to stay on until the dump valve is opened. The dump valve closes when it receives 5V from the Atmel micro, and it opens when that voltage is not present. Basically, I want the LED to be on when the 5V to the dump valve is present.

I plan to solder a wire into the dump valve's power line and connect it to the base of a BJT to switch the LED. The LED (C503B-RCN-CW0Z0AA1) will be powered by the 12V line to the BP pump, so I will need a current limiting resistor. I think I'll use a 2N3904 for switching, so I'll also need a base resistor. I've read that the base emitter current should ideally be 1/15 the load current. Is this correct for this application?


LED resistor

Input voltage = 12V
LED Vf = 2.1
2N3904 collector emitter saturation voltage = 0.2V
Target LED current = 20mA (or 0.02A)
(12V – 2.1V) - 0.2V = 9.7V to drop across resistor
9.7V =0 .02A*R
R = 485Ω (closest value 487Ω)
(9.7V^2) / 487Ω = 0.19W power dissipation of LED resistor


Base resistor

Input voltage = 5V
Base collector saturation = 0.65V
Target base emitter current 0.02A(1/15) = 1.3mA (or 0.0013A)
(5V – 0.65V) / 1.3A = 3.26kΩ (closest value 3.24kΩ)
(4.35V^2) / 3,240Ω = 0.006W power dissipation of base resistor


I need this to be reliable, so I hope someone will tell me if I have something wrong. I've attached an image of the schematic as it stands now.

[petehall347]

Re: Modifying a Blood Pressure Pump to Switch an LED

i would reduce current further to the led to stop it running hot .. but thats just me ... i like them dimmer than brighter .. and cooler ..
think if i remember right 3ma . i remember using 1.2k or 1.5k resistors at 12v . super bright led.s

[capwizard]

Re: Modifying a Blood Pressure Pump to Switch an LED

I would use MOSFET to Switch an LED. A positive input voltage and has an extremely high input resistance almost infinite, making it possible to interface with nearly any logic gate. You can even use fingers to activate this sensitive mosfet's switch.

MOSFET as a Switch
http://www.electronics-tutorials.ws/...or/tran_7.html

https://cdn.badcaps-static.com/pdfs/...bfd5ab2829.pdf

[petehall347]

Re: Modifying a Blood Pressure Pump to Switch an LED

this beer might be getting me confused with 30ma ... ignore me .. i will try remember ..

[budm]

Re: Modifying a Blood Pressure Pump to Switch an LED

So the dump valve COIL operates on 5V, 12V?
Why not put LED in parallel with the Dump coil and limit the current to the LED with resistor to about 10mA?
There should be a Transistor that drive the Dump coil.

[petehall347]

Re: Modifying a Blood Pressure Pump to Switch an LED

So the dump valve COIL operates on 5V, 12V?
Why not put LED in parallel with the Dump coil and limit the current to the LED with resistor to about 10mA?
There should be a Transistor that drive the Dump coil.

would the led survive back emf ? maybe there is a rectifier diode doing that anyway .its good practice ..
anyway i know bugger all about circuit design ..
as you were ..
sorry its the beer .

[budm]

Re: Modifying a Blood Pressure Pump to Switch an LED

There should be protection diode on the coil to protect the drive transistor already, otherwise the transistor would have been damage by now.

[petehall347]

Re: Modifying a Blood Pressure Pump to Switch an LED

There should be protection diode on the coil to protect the drive transistor already, otherwise the transistor would have been damage by now.

can take many years for that to happen .. i fix vintage audio mainly and relay drive circuits were often missing the flyback diode .. transistors lasted many years .. would have been much longer with the diode though .

[budm]

Re: Modifying a Blood Pressure Pump to Switch an LED

Without the snubber, the spike will spread to other circuit as well and you hear arcing/clicking through the amplifier.
I used to fix lots of TV and audio systems that were using tubes in the early of my electronics, tube can take more abuse than solid state device.

[capwizard]

Re: Modifying a Blood Pressure Pump to Switch an LED

" I want the LED to be on when the 5V to the dump valve is present." MOSFET is perfect candidate for this application, 5V from the Atmel microcontroller is A digital drive signal, the dump valve uses 12 v.

The benefit to use MOSFET (my suggestion: IRFZ 44 N-channel 60v 0.024Ohm 50A) is that in the future, you can add LEDs, Radio, MP3, 12v portable TV, drive another Dump valve and Buzz alarm.

[Winston Chadwick]

Re: Modifying a Blood Pressure Pump to Switch an LED

Sorry for the delayed response. Thank you all for the replies.

i would reduce current further to the led to stop it running hot .. but thats just me ... i like them dimmer than brighter .. and cooler ..
think if i remember right 3ma . i remember using 1.2k or 1.5k resistors at 12v . super bright led.s

From the data sheet: “For long term performance the drive currents between 10mA and 30mA are recommended.”

So the dump valve COIL operates on 5V, 12V?
Why not put LED in parallel with the Dump coil and limit the current to the LED with resistor to about 10mA?
There should be a Transistor that drive the Dump coil.

I had some trouble metering across the dump valve, but it looks like it has about 6.2V between the two wires going to the dump solenoid when the valve is closed. I considered running the LED from the 5V control line. I’m new to this, and I was worried about drawing too much current from the pump PCB. The brightness of the LED is a concern. I’ll have to test it at 10mA to see if it’s bright enough. I’ll attach a few pictures of the PCB.

" I want the LED to be on when the 5V to the dump valve is present." MOSFET is perfect candidate for this application, 5V from the Atmel microcontroller is A digital drive signal, the dump valve uses 12 v.

The benefit to use MOSFET (my suggestion: IRFZ 44 N-channel 60v 0.024Ohm 50A) is that in the future, you can add LEDs, Radio, MP3, 12v portable TV, drive another Dump valve and Buzz alarm.

I wanted to use a MOSFET originally, but they seem to be many times more expensive than a basic BJT. I eventually plan to design a PCB and modify hundreds of these blood pressure pumps, so component cost is a major concern. I like the idea of having headroom for other functions, but the pump is connected to a PC which handles all of the other user feedback. The light on the emergency stop button is the only thing that we need right now. If the BJT will work well for this application, I think I’d have a hard time selling the boss on a MOSFET.

[budm]

Re: Modifying a Blood Pressure Pump to Switch an LED

Well if you want simple, LED and resistor to limit to 10mA and two wires and it's done. 10mA is really low, adding 10mA load to that big dump valve drive circuit should be no problem at all.
If you want fancy, then you can use transistor to drive the relay so the relay contact circuit is completely isolated from your unit and can be used for switching anything you want.

[capwizard]

Re: Modifying a Blood Pressure Pump to Switch an LED

"I wanted to use a MOSFET originally, ............................... I think I’d have a hard time selling the boss on a MOSFET"

You don't need any MOSFET parts just add LED and resistor to the existing MOSFET right on the PCB board, your boss will be extremely happy.

budm, you are faster than me!.................

ZXMN3A01F
https://cdn.badcaps-static.com/pdfs/...81ad06d7fb.pdf
FDS8984
https://cdn.badcaps-static.com/pdfs/...a9f84fd7d3.pdf

_________________________
Inauguration.... I bet ju.............the First Lady gonna wears white dress and Trump will dresses black !

[Winston Chadwick]

Re: Modifying a Blood Pressure Pump to Switch an LED

Thanks again for your help.

I made a schematic (see attached pic) to get a better idea of what's happening in this circuit.

I tested the LED at 10mA, and the brightness is acceptable.

My current plan is to desolder the control signal wire and add the positive wire to the same hole. There is a conveniently unused grounded through hole nearby that I will use for the negative lead.

"I wanted to use a MOSFET originally, ............................... I think I'd have a hard time selling the boss on a MOSFET"

You don't need any MOSFET parts just add LED and resistor to the existing MOSFET right on the PCB board, your boss will be extremely happy.

budm, you are faster than me!.................

ZXMN3A01F
https://cdn.badcaps-static.com/pdfs/...81ad06d7fb.pdf
FDS8984
https://cdn.badcaps-static.com/pdfs/...a9f84fd7d3.pdf

_________________________
Inauguration.... I bet ju.............the First Lady gonna wears white dress and Trump will dresses black !

To get any benefit from the the existing MOSFET I would have to solder the positive LED lead somewhere between the negative lead from the solenoid relay and the drain of Q1(FDS8984), correct?

That would mean my options for soldering would be the negative pad for the relay, the anode of D1, or the drain pins of Q1.

It looks like soldering a wire there would require removing several components to have access to that area. If I powered the LED from this area, would I be able to draw more current?

The current plan would have the positive wire attached to the gate side of this MOSFET.

[budm]

Re: Modifying a Blood Pressure Pump to Switch an LED

Just solder the two wires of the LED assembly to the two solder pads where the Valve is, that is too hard to get access to those two pads?

[budm]

Re: Modifying a Blood Pressure Pump to Switch an LED

BTW, you FDS8984 D1, D2 marking are wrong in your SCH.
5 & 6 are for Q2
7 7 8 are for Q1

[Winston Chadwick]

Re: Modifying a Blood Pressure Pump to Switch an LED

BTW, you FDS8984 D1, D2 marking are wrong in your SCH.
5 & 6 are for Q2
7 7 8 are for Q1

Thanks I'll correct it.

[Winston Chadwick]

Re: Modifying a Blood Pressure Pump to Switch an LED

Just solder the two wires of the LED assembly to the two solder pads where the Valve is, that is too hard to get access to those two pads?

I've attached pictures that show my access to the area when fully assembled. The dump valve board is through hole soldered to 12 pins on the main PCB. The holes here are a bit undersized, so it's a pain to desolder even with good equipment. If possible, I would like to do the modifications without having to desolder all of those pins, and I think soldering wires to the existing pads without removing the dump valve board or wires would be tricky at best.

What do you think about desoldering the positive dump valve wire, soldering in a new wire to supply a small add-in PCB, and then soldering the dump valve wire to the pcb? This way I could use your idea to isolate the relay with a transistor. I've attached a schematic to illustrate this. Is this what you meant by, "you can use transistor to drive the relay so the relay contact circuit is completely isolated from your unit"?

Edit: It looks like pins 1, 5, and 6 of Q1 are tied together, not floating like I had in my schematic. That makes more sense.

[budm]

Re: Modifying a Blood Pressure Pump to Switch an LED

https://www.badcaps.net/forum/attach...6&d=1485190834
Well, that added circuit add another EC junction drops in series with the Valve coil, and the LED/resistor is n the path of the EB junction bias, so it will need enough EB bias current to turn on that added transistor to be fully on. Also that does not allow to add the drive relay which the power supply of the unit has to have extra current to able to drive the relay coil.
BTW, what is the resistance of that valve?
Also, there is not Gate resistor for that Q2 MOSFET?
What pin is that R4 connected to btw to that 555?

[capwizard]

Re: Modifying a Blood Pressure Pump to Switch an LED

Is this gonna work?