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555 timer and transistor problem

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jasonspinmaster

New Member
i have made up a circuit with an optic sensor, LM555 timer and transistor.
i am getting a pulse, but the transistor is staying on can some one help please
 
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Roff

Well-Known Member
Does the transistor go off when you remove the light source from the phototransistor?
 

jasonspinmaster

New Member
the transistor should switch ON when the light is blocked.
i am using a nail which is 1mm wide the slot is 0.75mm wide
 
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Roff

Well-Known Member
Your circuit is a not a pulse generator. If all your parts are good, the transistor will turn on when the voltage on pin 2 is below Vcc/3, and it will be off when the voltage is above Vcc/3.
You could have other design errors.
1. You don't show a power supply. What is the voltage (Vcc)?
2. What is the resistance of your load (the coil)?
 

Roff

Well-Known Member
You should be driving the base of the 2N3055 with at least 120mA (Ic/10). This means the base resistor should be about 75 ohms, unless you are using a CMOS 555, which can't source 120mA, so you would need to switch to a bipolar 555 before proceeding.
You also need some capacitors from Vcc to ground, with short leads, as close to pins 1 and 8 as possible. I would use a 100nF (0.1uF) ceramic cap in parallel with an electrolytic, 100uF or so.
If you want to create a fixed length pulse, you need an RC timing network.
Where did you get this schematic?
 

jasonspinmaster

New Member
my timer is a LM555, it is the right one?????
i got the schematic from some one on this web page. could you be kind enough as to send me some circits that works as the last 6 months have been hell.
i only need the pulse to fire inconjuction with the nail (trigger)blocking the light
 
Last edited:

Roff

Well-Known Member
jasonspinmaster said:
my timer is a LM555, it is the right one?????
i got the schematic from some one on this web page. could you be kind enough as to send me some circits that works as the last 6 months have been hell.
i only need the pulse to fire inconjuction with the nail (trigger)blocking the light
Do you want a pulse with a defined width, or do you want the transistor to be on whenever the nail blocks the light? With your schematic, the transistor should be on whenever the nail doesn't block the light.
Do you have a lot of ambient light? That will make it more complicated.
 

audioguru

Well-Known Member
Most Helpful Member
There is a duplicate or triplicate thread about this circuit. I corrected it by changing the 2N3055 for an MJ2955 and moving the coil to ground. But he says his MJ2955 keeps on blowing up.
 
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