# Tactile Sensor circuit

Discussion in 'Homework Help' started by AD633, Jul 6, 2013.

Jun 22, 2013
96
1
T circuit represents a tactile sensor to turn on / off a relay to turn on / off a load of greater power.The tactile sensor consists of two separated metallic contacts about 2mm.When you place your finger on one of these contacts circulates certain current between them.

hFE (T1) = 300 hFE (T2) = 220 Vgama VCEsat = 0.7 V = 0.5 C

Considering that the relay needs a current of 70 mA to close the contacts and that in this case both transistors should be saturated.Determine

1)hFE min for transistor T2

2)The minimum current that should flow in the tactile sensor when ativated(that is pressed by the finger)

1)How can i determine the mininum hfe of T2.The hFE of T2 must a have so that we have Ic2 sat =70 mA

I have tried to do $12 V=Vcesat1+1kOhm*(Ic1sat)+VBE2 sat

12 V=0.5 V+1kOhm*(70 mA)+VBE2sat$

but i am not going nowhere form here.

Where should i start for determing this?

Thanks

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2. ### antonv Member

Nov 27, 2012
149
27
I think you are making it too complicated. What is hFE?

Jun 22, 2013
96
1
hFE represents the gain of the transistor. I have to determine the minimum gain for the transistor T2 which ensures that the collector current of transistor T2 is 70 mA.Therefore the transistor T2 has to be in saturation.

I also thought about doing something like $12V-VCE1sat=VE1

VE1=11,3 V$

Therefore $Ib2=(11,3V)/(1kOhm)=11,3 mA$

In the ative region $Ic=hFE*IB

70mA=hFE*(11,3mA)
hFE=6,19$

Which i dont think its correct

Thanks

Last edited: Jul 7, 2013
4. ### antonv Member

Nov 27, 2012
149
27
I am a little confused. Where does the following come from in your first post:

hFE (T1) = 300 hFE (T2) = 220 Vgama VCEsat = 0.7 V = 0.5 C

Jun 22, 2013
96
1

hFE (T1)=300 is the typical value for the gain of transistor T1
hFE(T2)=220 is the typical value for the gain of transistor T1
VCEsat of T1 is 0.7 V

Jun 22, 2013
96
1
What i want to determine is minimum value of the gain for transistor T2 in order to have T2 in saturation.