Touch Alarm Problems

Thread Starter

kazz

Joined Dec 27, 2013
10


Can anyone suggest or help me how to convert the circuit with one touch plate only, Using the same components? I'm still new to electronics, and i just understand a little bit on this circuits and functions of each components. BTW im using 3-9 V, is it OK ryt?

Thanks for Suggestion/Help.
 
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Alec_t

Joined Sep 17, 2013
11,755
I don't see any way to use a single touch plate if you can use only those components.
Any simple circuit using a single touch plate is likely to be triggered by interference.
 

#12

Joined Nov 30, 2010
18,210
I've used a jfet to pick up finger hum. It would suffer from not being universally dependable as a retail product, but it might work for you. Google fet touch switch.
 

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Thread Starter

kazz

Joined Dec 27, 2013
10
I don't see any way to use a single touch plate if you can use only those components.
Any simple circuit using a single touch plate is likely to be triggered by interference.
you can add any components as long it have a atleast 3 transistors,....and the easily trigerred, im trying to fix it by changing the values of the resistors near in the plates. Am i doing it right?
 

Thread Starter

kazz

Joined Dec 27, 2013
10
I've used a jfet to pick up finger hum. It would suffer from not being universally dependable as a retail product, but it might work for you. Google fet touch switch.
your circuit was good, thanks for it, i will try this after my project,.... .but my project neede d atleast 3 transistors
 

Alec_t

Joined Sep 17, 2013
11,755
Since this is appears to be a Homework project, we won't give you a complete solution; but consider how you would modify your circuit around T1,T2 to incorporate #12's suggestion of using a jfet (e.g. a 2N3819).
 

Thread Starter

kazz

Joined Dec 27, 2013
10
Since this is appears to be a Homework project, we won't give you a complete solution; but consider how you would modify your circuit around T1,T2 to incorporate #12's suggestion of using a jfet (e.g. a 2N3819).
if im going to change T1 and T2 experimentally, it cost lots of expenses which might damage other components...and i cant try this to simulation since it needs to touch to work. What if i change the value of resistors? but i dont know which of the resistors.
 

MrChips

Joined Oct 2, 2009
22,113
Yes, the simplest circuit begins with an N-FET or MOSFET as the first stage as #12 suggested.

Bias the FET correctly and you can pick up 50/60Hz line frequency interference whenever you touch the gate.
 

MrChips

Joined Oct 2, 2009
22,113
Actually, a FET is easier to understand than a BJT.

A BJT is a current device. The base-emitter current controls the collector-emitter current.

A FET is a voltage device. The gate-source voltage controls the conduction channel and hence the resistance of the channel. The gate input impedance is very high and hence is more sensitive for applications such as touch sensors.
 
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Thread Starter

kazz

Joined Dec 27, 2013
10
You can simulate touch, by applying a voltage (50/60Hz hum signal) via a high resistance (representing a finger).
what simulation can i try? with a complete components, im just using circuit wizard but lack of components like the transistors,....any good simulators that have all components in the given diagram?
 
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Alec_t

Joined Sep 17, 2013
11,755
what simulation can i try? with a complete components, im just using circuit wizard but lack of components like the transistors,....any good simulators that have all components in the given diagram?
I use LTspice; a free download from Linear Technology, with extra libraries of models freely available from the Yahoo LTspice User Group.
 

Thread Starter

kazz

Joined Dec 27, 2013
10
@Alec_t, i have a question, why does the a 0.3W speaker is not producing a sound when i supply a 9volts source,....it just vibrating, but when i supply a 3V source it make sounds,.. i dont see whats the logic of the speaker,....And about the other component i try to simulate it and change it values, it seems nothing change, i cant point out whats the real purpose of each of the resistors and capacitors

thanks for the reply
 

Alec_t

Joined Sep 17, 2013
11,755
why does the a 0.3W speaker is not producing a sound when i supply a 9volts source,....it just vibrating, but when i supply a 3V source it make sounds
Whether or not the circuit works seems to depend critically on component values selected, particularly on the impedance and voltage of the battery powering it.
 
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