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# transistor switch questions

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#### daviddoria

##### New Member
I am trying to attach 2 handsets to 1 phone so i can have them in different parts of the room. I am trying to make a switch to simulate taking the phone off the hook. I figured out that there are 7 wires... to simulate "ON HOOK", there are
Bl to R
and
Gr to Y
connections, with Y not connected to Br, and W not connected to G

for "OFF HOOK"
Y to Br
and
W to G

How can i make all that happen by switching only 1 switch? Can I use transistors to do it? I dont have MOSFETS, as suggested below.

lmk if you know how i can do this.

thanks
david

The answer is yes and yes.

Sounds like you can use mechanical relays for this. I would first draw your circuit with wire connections connected to open and closed contacts (generic switch contacts), Draw that circuit for the "ON-HOOK" state and then draw another one in the "OFF-HOOK" state. Recongnize the similarities and ON/OFF nature of the contacts and then find a mechanical relay with enough contacts (or more than one if needed). Then one signal can turn on or off all of your relays thus putting the phone in the ON/OFF hook positions. Your drive signal can be just one switch hooked to each relay coil and the power source (-48V for phones?) make sure the relay coils are rated correctly. I think you could get this working the quickest.
Small signal relays are relatively cheap and easy to use.

OR

You can use transistors.. I would use MOSFETS as the switches - that way I don't have to think about the base currents. take your switch diagrams and just wire in your FETS.. use N-channel for one of the states (ON or OFF) and use P-channel for the other(complimentary state).. There are LOTS of ways to get this wired up to work. One drive signal (one switch for ON/OFF hook control) can drive all of them.

For both methods, be sure you know you can safely pass the currents flowing through the switches and make sure the parts are rated to be run at the supply voltages you will be using.

FETS are smaller and cheaper than mechanical relays but may require more of them to solve your problem.

OK, what happend to the original handset? is it left on or off the hook?
You will also need some extra wires to go to your remote handset for the switch, can you do that?
If I was doing that I would replace the hook switch inside the phone with a relay which has a similar contact arrangement. Multi contact relays can often be salvaged from old phone equipment.
I would then wire a PTT (push to talk) switch to each handset, just like the switches in two way radio sets. If pressed, the switch would activate the relay which in turn simulates the off hook condition.
The PTT on each handset are in parallel so either being activated will take the phone off the hook.

But, I would still need a looooong arm to dial from out from the remote handset :wink:

Klaus

both handsets will be left off the hook... i have completely removed the normal switch from the phone.

the extra wires to the other phone is no problem

dialing is no problem, receiving the calls is the idea.

still kinda looking to learn to use transistors better, i currently only know how to use NPN as amplifiers... not sure about switching however... something about saturation... ? lol

still kinda looking to learn to use transistors better, i currently only know how to use NPN as amplifiers... not sure about switching however... something about saturation... ? lol

If you know how transistors work as amplifiers then you should have no trouble understanding how they work as switches.

When transistors are used as switches they operate between saturation and cut-off, which is determined by the base resistor . At saturation the transistor is delivering max current to a connected load, in other words its acting similar to a closed mechanical switch. Cut-off is just the opposite.

But there are a few important things you need to know before you can design a circuit using a transistor switch. The key things are:

1. Power supply voltage
3. the beta of the transistor
4. Max current ratings of the transistor

Its obvious that the data sheet for the particular transistor you are using would really come in handy.

Then you have to do some math to calculate the base resistor required for saturation. As a rough guide: For saturation the Beta of the transistor multiplied by Base current should be equal to collector current.

Beta x base current = collector current

OR

Ic = Beta x Ib

As a design example: Beta of a particular transistor = 100

Power supply = 12 volts

Base resistor = ?

First find load current, this will also be collector current ( Ic ).
Since at saturation nearly all supply voltage appears across the load ( remember the closed switch analogy) the current thru the load will be:

12/50 = 0.24 amps

Then find Base current ( Ib ) required for saturation using a derivitive of the given formula:

Ib = Ic/Beta

= .24/100

= .0024 amps

Neglecting the small voltage drop of the base-emitter diode, the base resistor will be:

Rb = 12/.0024

= 5000 ohms or 5 K

This is the max value of resistance that will give saturation of this particular transistor, you can use a smaller value and it will still allow for saturation current ( within limits of data sheet ratings ), so a good standard value would be 4.7 K ohms.

Sorry for the long winded reply, but you asked for it . :wink:

awesome dude

i gota measure lots of little currents now, but sounds like that will work.

thanks
david

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