Hi,

I have come a cross the attached counter circuit while I was searching for my stopwatch timer project. the same principle of time-multiplexing is used in this circuit and mine. I would like to know the purpose of the 4.7kΩ resistors that are connected to the base of the transistors, are they for some protection purposes, or is it related to the transistors ? also, how was the resistor value calculated ?

The transistors are used as switches.


Thanks in advance,

 

That's an easy one. A transistor emitter-base junction is just a diode, and theoretically any voltage greater than ~0.6v will push unlimited current through it. The base resistors limit the current to about 1mA.

The resistors were probably sized with a guess at the gain of the transistors, say 50 or so. Thus the current through the LED segments would be 50mA max--and I wonder if it's enough! That has to drive all segments of one digit at once; the per-segment current would be limited by R4-R10.

By the way, the pic16f628a is pin-compatible with the pic16f84a, but it has more features and it's cheaper.

 

Thanks John for the response.

> Is there a formula to calculate the resistor value ?

> I will have a look at the pic16g628, thanks for the advice

 

Normally as a rough rule of thumb I'd go for 1/10th the current that the transistor would be powering. Obviously you wouldn't want to go over the current capability of the PIC pin. 500 ohms would give you about 10mA which should be good.
More saves power but means the transistor might get hot because it is only partly switched on.

 

I found this formula in another website: Max base Resistance = (Vdd - 0.65) / (Ic/hFE)
but I am not sure if it is the correct one. can anyone approve ?

 

here is a 4 digit multiplexed 7 segment display I recently just did not too long ago, http://forum.allaboutcircuits.com/attachment.php?attachmentid=18943&d=1272466284

It might help enlighten some of your questions about component values....

B. Morse

 

Thank you guys, but I still don't get it.

Isn't there an accurate way of measuring the resistance required, or at least the range of resistance values that can be used ?

 

You can look at the data sheet and see what the current limits are.

You know the voltage you are working at, so ohms lay will tell you how much resistance you need.

If you are using 5v and you have a IC that can handle a max or 20mA on any input pin,
using ohms law:
5v * .02 = 100 ohms
So you need to have at least 100 ohms in resistance "protecting" that pin

Some devices, like a 555, use the resistance between 2 pins to "tell" the chip something

For instance if a timer counts 1second per ohm, you would use 60 ohms to "tell" the timer to count for 60 seconds.

There is also impedance matching. Where a device may need a certain amount of resistance on the input or output for accuracy.

The truth is in the datasheets.