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- Nov 28, 2011
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That's a trick question, because you haven't told us how much current the 6V motor draws!
Also, you will lose most of your 6V because (a) using the transistor as an emitter follower like that, you will get about 0.8V lost across the base-emitter junction, and (b) when sinking current, the 555's output will not pull down to 0V; actually if it's sinking 200 mA it will only pull down to typically 2.5V.
Your best bet would be an NPN emitter follower buffer driving an NPN switching transistor connected as a common emitter saturated switch.
The emitter follower can be any small NPN that has a maximum collector current that's comfortably greater than the required base current for the main transistor. A 2N4401 or a BC337 would be suitable.
The main switching transistor needs to be an NPN, rated to carry the current required by the motor. Aim for a transistor with as much current gain as possible; this will allow you to keep the base current relatively low. But you can't use a Darlington; it will drop about 1.5V collector-emitter, leaving only 4.5V for the motor.
Here's how to connect them.
555 pin 3 to base of emitter follower transistor.
Collector of emitter follower transistor to the 555's positive supply rail.
Emitter of emitter follower transistor through a current limiting resistor to the base of the main transistor.
Emitter of main transistor to common 0V (negative supply) rail.
Collector of main transistor to motor and reverse-connected diode (same as drain connection of MOSFET).
Motor connected between collector of main transistor and a positive voltage rail.
The current limiting resistor will determine the base current into the main transistor. This current must be high enough to saturate the transistor fully. You can try a 6.8 ohm resistor to start with.
A bipolar transistor is not well-suited to this application. High-current bipolar transistors have fairly low current gains, so they need a lot of base current to saturate them properly. Even then, at currents of a few amps or more, you can lose half a volt or more across the transistor's collector-emitter. A MOSFET with low ON-resistance is a much better choice, especially at slow switching frequencies like this.
You would probably be better off using a boost converter to generate a 12V rail for the 555. It would then have enough voltage to fully saturate a big ol' grunty MOSFET. The boost converter will waste a little bit of power, but much less than the power you would waste feeding hundreds of milliamps into the base of a bipolar transistor.
Also, you will lose most of your 6V because (a) using the transistor as an emitter follower like that, you will get about 0.8V lost across the base-emitter junction, and (b) when sinking current, the 555's output will not pull down to 0V; actually if it's sinking 200 mA it will only pull down to typically 2.5V.
Your best bet would be an NPN emitter follower buffer driving an NPN switching transistor connected as a common emitter saturated switch.
The emitter follower can be any small NPN that has a maximum collector current that's comfortably greater than the required base current for the main transistor. A 2N4401 or a BC337 would be suitable.
The main switching transistor needs to be an NPN, rated to carry the current required by the motor. Aim for a transistor with as much current gain as possible; this will allow you to keep the base current relatively low. But you can't use a Darlington; it will drop about 1.5V collector-emitter, leaving only 4.5V for the motor.
Here's how to connect them.
555 pin 3 to base of emitter follower transistor.
Collector of emitter follower transistor to the 555's positive supply rail.
Emitter of emitter follower transistor through a current limiting resistor to the base of the main transistor.
Emitter of main transistor to common 0V (negative supply) rail.
Collector of main transistor to motor and reverse-connected diode (same as drain connection of MOSFET).
Motor connected between collector of main transistor and a positive voltage rail.
The current limiting resistor will determine the base current into the main transistor. This current must be high enough to saturate the transistor fully. You can try a 6.8 ohm resistor to start with.
A bipolar transistor is not well-suited to this application. High-current bipolar transistors have fairly low current gains, so they need a lot of base current to saturate them properly. Even then, at currents of a few amps or more, you can lose half a volt or more across the transistor's collector-emitter. A MOSFET with low ON-resistance is a much better choice, especially at slow switching frequencies like this.
You would probably be better off using a boost converter to generate a 12V rail for the 555. It would then have enough voltage to fully saturate a big ol' grunty MOSFET. The boost converter will waste a little bit of power, but much less than the power you would waste feeding hundreds of milliamps into the base of a bipolar transistor.
