what resistor to use for transistor base? Doing a touch circuit with 2 transistors

Greetings all,

I am doing a touch switch which will be activated when the touch plate is touch, however i do not know why is the resistor used is 100k ohms. As i will be using another surface mounted transistor, how do i know what value of resistor to use?

i have found a transistor base resistor calculator online: http://kaizerpowerelectronics.dk/calculators/transistor-base-resistor-calculator/

i will be using the FZT651TA, NPN Bipolar Transistor for the circuit.
Datasheet:  http://docs-asia.electrocomponents.com/webdocs/0026/0900766b800261a4.pdf

Can somebody tell me what resistor to use for this transistor?

Thanks in advance

Picture of what resistor to use for transistor base? Doing a touch circuit with 2 transistors
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iceng4 years ago
Ok, I like a single TO-92 MOSFET...
The zener leaks a micro current keeping the gate low and
zener voltage can be anything from 8Vz to 15Vz ..
verence4 years ago
The 100k protect the transistors in case there is a shortage across the touch plate. The resistance of a finger will be higher than 100k, so the value is not critical at all.
ashbite1 (author)  verence4 years ago
Thanks! Will it be possible to use less than 6V too? I'm considering using a surface mounted LED too.
The form factor of the LED (SMT, through-hole, 3mm, 5mm, round, square etc.) doesn't matter at all. The colour does as the forward voltage depends on the colour (red about 1.6V, green/yellow about 2.2V, blue/white 3V and up - read the data sheet). You will need at least 1.4V for the two base-emitter diodes in the transistors plus the forward voltage of the LED plus a bit to drop on the current limiting resistor. So with a red LED you need at least 3V. All the numbers from top of my head, they may not be 100% exact but good enough as a rule of thumb.
ashbite1 (author)  verence4 years ago
so its actually okay to use around 4-5V batteries if im using the red LED with 1.7V? as i will be using button batteries
Hmm, let's do the math...

You'll need about 1.4V for the two B-E diodes: 4V - 1.4V = 2.6V. (worst case, batteries empty). The Uf (forward voltage) of a red LED is about 1.6V (give or take some ten millivolts), so about 2.6V - 1.6V = 1V has to be dropped at the current limiting resistor. Let's say you want to have a current of at least 20mA through the diode, you would need a resistor of 1V / 20mA = 1V / 0.02A =50Ohm (try 47Ohm).

You could try a low current LED, they need only about 3mA, so the resistor should be around  1V / 0.003A = 330Ohm.
It depends on your base current and your voltage I would go with 10K.

I think the datasheets are better here

100K should work fine.
ashbite1 (author)  steveastrouk4 years ago