# Will I be able to drive LED with attached analog circuit ?

• 19th August 2019, 16:40
AzharElectronics
Will I be able to drive LED with attached analog circuit ?
Hi,
I have Aerospace customer released analog circuit as attached with output voltage of 3.5V max and output current max 3.5mA.
Will this drive customer LED placed at 65inch away from this analog circuit unit? My worry is with LED's Forward Voltage and Forward Current. Any things that can be modified in existing design without form fit (PCB layout) changes would be helpful. Earlier design was consider a controller TTL logic and now there is customer requirement change for connecting LED indicator Lamp.
D1 is reference LED used for simulation.
R4 and R5 are wire resistance calculated using reference for 26 AWG as 0.134 Ω/mtr ≈ 0.28 Ω for 2 meters of wire.
• 19th August 2019, 19:08
wwfeldman
Re: Will I be able to drive LED with attached analog circuit ?
i think it will work
but the LED is always on, and there are simpler circuits to just turn on the the LED
at 3.5 mA the LED will not be very bright.
• 19th August 2019, 22:04
albbg
Re: Will I be able to drive LED with attached analog circuit ?
The LED wiring is not a problem, it will be always negligible with respect to R3
Form the datasheet of the LED and the value in your schematic I can expect a current IF=2 mA and VF=3 V roughly. With a so low IF the led will have a luminosity of about 1/10 with respect to that at the nominal current (20 mA).
I can suggest you to use a low current LED. You can have a look to: https://www.vishay.com/leds/low-current/
• 19th August 2019, 22:22
betwixt
Re: Will I be able to drive LED with attached analog circuit ?
It will work but I would use a bipolar transistor in this circuit. My reasoning is that to use the MOSFET as a switch its gate voltage needs raising more than knee voltage above the source pin. In a follower circuit like that, the gate therefore has to be at least 2V higher than the voltage dropped in R1 and the LED.

If you use an NPN bipolar transistor the voltage will only have to be about 0.6V higher. You can utilize the current gain of the transistor without needing so much drive voltage.
You could also consider reversing the operation, connecting the left side of R3 directly to 5V and using a PNP transistor with it's collector to ground as a shunt across C1.
Ignore the wiring resistance, it contributes only about 0.1% to the LED current drop.

Brian.