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i have to make a portable handly lie detector but i was not able to get a 2n3904.What substitute can i use?I have searched a lot and get to know that 2n2222,2n4401,2n2907,bc547 can be a replacement but i want to know a perfect substitute for 2n3904 transistor.

This is the circuit we're dealing with: enter image description here

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    \$\begingroup\$ Do you know which characteristics of the transistor are important? \$\endgroup\$ – Ignacio Vazquez-Abrams Oct 13 '16 at 13:40
  • \$\begingroup\$ not really but the thing is that whenever i use that lie detector or the voice reader the light should light up.It includes 10K resister 47K resister 470 resister 1M resister x2 and 47K& VR and x3 2n3904 transistors. \$\endgroup\$ – vaidehi Oct 13 '16 at 13:56
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    \$\begingroup\$ Go look up a data sheet for the 2N3904. Notice that most values (leakage, hfe, etc) have a range of possible values. So there is no such thing as a "perfect" replacement - only one which is generally the same. Now look at the data sheets of the potential replacements, and pick one with a good match. \$\endgroup\$ – WhatRoughBeast Oct 13 '16 at 14:10
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    \$\begingroup\$ Forget about the "perfect" replacement, often that is not needed. I'd say that a circuit which would only work with a 2n3904 (or a "perfect" replacement) is a very bad design. \$\endgroup\$ – Bimpelrekkie Oct 13 '16 at 14:21
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    \$\begingroup\$ 2n2907 is PNP, so... no, unless you want to redesign the circuit. The other one should do, but you might have to trim some values. \$\endgroup\$ – Sredni Vashtar Oct 13 '16 at 15:16
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First, find the datasheet of the device you want to replace. That's the easy bit.

Now the harder bit. Obviously the replacement must be of the same type (bipolar NPN) and preferably designed for the same applications (general purpose amplifier). But beyond that the datasheet may provide a lot of parameters. The chances of of finding an identical substitute are slim, so you will have to decide which numbers are important, and how closely they must match.

Top of the list is Absolute Maximum Ratings. These must never be exceeded, or the device will fail. The perfect replacement would have the same or higher ratings, but they could be lower provided that your circuit doesn't come close to them.

The 2N3904 is rated for a maximum of 40V between Collector and Emitter, and 200mA Collector current. Your circuit is only powered with 9V, and the current passing through each transistor is limited to less than 20mA, so any replacement that can handle eg. 20V and 100mA will be fine.

Next is Electrical Characteristics. Depending on the application, some may be more important than others. Your circuit compares a slowly changing DC voltage to an adjustable reference voltage, and switches on one of two LEDs. So you don't have to worry about high frequencies, AC signals, or switching speed, and you have plenty of voltage available so saturation voltage is not critical.

That leaves DC current gain as the essential parameter that must be duplicated. But confusingly, the datasheet doesn't provide a single number. Instead you are given a range with minimum, maximum, and perhaps 'typical' values for different currents and voltages. Why so many numbers? The reason relates to how transistors are made. The diffusion process that creates its elements is not precise, so every transistor that is produced has different characteristics. The manufacturer may sort them and label similar units with the same part number, but they don't throw away any that aren't identical because that would make the 'perfect' remainders too expensive.

At 10mA Collector current the 2N3904's current gain could be anywhere between 100 and 300. Any substitute that is within or close to that range should be fine (assuming the circuit was designed to accommodate such a wide variation). In your circuit higher gain is better, but 100 is plenty enough. So just look for a substitute that has HFE of at least 100 at an IC of around 10mA, and you should be OK.

Finally, before settling on a particular part you might want to check its Packaging - case style, lead designations etc. The 2N3904 comes in a TO-92 case with leads in the order C,B,E. Some equivalent transistors have the opposite pinout (E,B,C) and some are B,C,E. This won't be a problem so long as you are aware of the difference, and refer to the datasheet of the part you are using when wiring up the circuit.

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Any of the substitutes you already mention will work:

  • 2N2222
  • 2N4401
  • 2N2907
  • BC547

Really any general-purpose small-signal transistor will work for this circuit.

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The colours are actually reversed.

  1. SWAP Red Green
  2. Use any Darlington NPN's (all 3)
  3. Change R4 from 470 to 100

Result is 10 mA ~ 12 mA with 9.0V Vbat

  • slightly dim zone with both ON in between < 10mA
  • if Red is too dim from raising threshold ( too low skin R )

    • change R1 from 1M to ~470k
  • play with my Java Sim. here

  • these validate my changes
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