Electronic – Determine suitable resistor for the base of transistor as switch (NPN)

• What Oli says

• Plus

  The parallel port notionally uses TTL levels. The actual levels accepted my vary in modern implementations. Theoretically, if you are driving a pin which is set to input then the "high" level can be as low as 2 Volts.

  If the pin you are driving is feinitely an input you should be able to drive it with a 555 directly (see below re voltages).
  This '555 to parallel port interface appication seems to do exactly what you want. See circuit diagram below and do look at the above referenced page.

• If you are using 2 x non rechargeable AA cells the available voltage will be about 3V max when the batteries are new and as little as under 2 Volts when the battery is almost dead. A circuit could be built which operates correctly on new batteries BUT this will get marginal as batteries are depleted.Driving the port directly with 2V is marginal at best.

• Using a transistor as a "pull down" allows th port to float high when the transistor is off. Even if you only have 2V available the pin can float to whatever voltage is available from the port - say 5V. Expecting TTL pins to float high without a pullup is bad design practice but worked well with real TTL and was commonly done. Wity modern implementations in aCMOS master IC the pin MAY NOT float high properly without a pullup resistor. Finding a pin on the port which i ALWAYS high (nominally 5V) and connecting a say 10k resistor from there to the input pin may help.

• You say you are using a "555". Is it also running from the same 2 batteries? Some 555's require about 4.5V minimum to operate whole others will operate on less than a volt. Check the spec sheet to be sure what voltage the device that you have will operate on.

You really should post a schematic instead of all the hand waving. However, there is one obvious flaw in your description. One side of the relay could should be connected to the collector of the transistor as you say, but the other side must be to the power supply, not ground.

When the transistor is off, the collector is effectively not connected or left floating. When the transistor is turned on, it will pull the collector low. Since that end of the coil will be low, the other end must be high for there to be voltage accross the coil so that the relay can turn on.

Here is how you should hook things up:

Don't leave off D1. It is there to catch the inductive kickback from the coil when it is suddenly shut off. Without it, the coil will generate high voltage and fry the transistor.