# Claulating the resistance for the IR Flame Photodiode sensor

Hi I wish to make a IR Flame sensor. I have made a schematic as below.

But I could not determine the values for R(x) & Rpot(y). I have tried x = 10K, y=10K , x=1K , y=10K,etc (randomly). But I could not fully control the output using the potentiometer.

I need to fully control the output using the potentiometer. Please help me calculate the values of x and y. Also let me know if anything else cold be added for the better functioning of the sensor.

An LM741 is severely limited in a lot of applications: -

• Minimum recommended power supply rails are +/- 10 volts
• Input voltage range is typically from -Vs + 2 volt to +Vs - 2 volt
• Input offset voltage is typically 1 mV (5 mV maximum)
• Input offset current is typically 20 nA (200 nA maximum)
• Input bias current is typically 80 nA (500 nA maximum)
• Input resistance is typically 2 Mohm (300 kohm minimum)
• Typical output voltage swing is -Vs + 1 volt to +Vs - 1 volt
• Guaranteed output voltage swing is -Vs + 3 volt to +Vs - 3 volt
• Supply current is typically 1.7 mA (2.8 mA maximum)
• Noise is 60 nV/sqrt(Hz) for LM348 (quad version of 741)
• GBWP is 1 MHz with a slew rate of 0.5 V/us

So it looks like you are falling at the first hurdle by not supplying the 741 sufficient supply voltage.

I need to fully control the output using the potentiometer.

I have no idea what that is meant to imply. Fix the power problems then decide what functionality you are desiring (important to specify this succinctly and with accuracy).

• Can I replace LM741 with LM358 ? – Dasaradh S K Jan 19 '17 at 15:55
• Either Op Amp can work, but LM358 has better sensing near 0V. Which PD sensor? Hopefully a black one with daylight blocking filter. then distance from flame to PD=?, determines current from PD in uA. Voltage drop may require 1M. Then for linear detection of flame , reduce the gain by adding Negative feedback starting with 100x the Pot value from output to -ve input. Then adjust from there. 331 R may be near limit on current so try 470 depending on Vcc. Add series R's on both sides of Pot to reduce sensitivity after you finalize position. – Tony Stewart Sunnyskyguy EE75 Jan 19 '17 at 16:11
• @DasaradhSK I reckon so but you still have disclosed what you mean by "fully control the output using the potentiometer". It's possible that when you come clean you need a rail to rail op-amp and the 358 won't fit the bill. – Andy aka Jan 19 '17 at 16:16