LED clock Jumbo Digits

If you increase the supply for the LEDs to 7.2V (well, greater than that, because you need to account for the voltage drop across the CE junction of the transistor as well as across the current-limiting resistor, so let's call it at least 8V), you are increasing the voltage at the emitter of the transistor.  Normally, you would disable the PNP transistor with a HI logic level, which means the base and emitter of the transistor would be at the same potential, not allowing for current to flow from the emitter to the collector or vice-versa.  When you increase the voltage at the emitter, even with a HI (5V) at the base, the PN junction will be forward-biased.  With a HI logic level on all of the LED driving pins, there SHOULD not be enough potential difference to forward bias the segments of the display.  This is definitely not ideal.  Also, there will constantly be a current flowing (or attempting to flow) from the source, through the emitter-base (PN) junction of the transistors, back through the output pins on the micro-controller, even when the pin is at a HI logic level, attempting to turn "off" the transistor.  I don't know what (if any) the potential for damage is here to your micro-controller in this configuration (pin configured as output/low impedence, with HI logic level at the output, with a higher potential attempting to flow back through the pin to the lower potential).

Thanks Indeed; true; as the 7.2 flows through the big digits; the voltage would somehow endanger the PIC chip. although by concept, LED will cause a drop in voltage as it passes through...

so what would be the safe way to replace these small digits with the bigger one's ? can you please give some hints ?

One possibility is to use an additional drive transistor, using the 5V logic level from the microcontroller to switch the higher voltage on/off to the base of the transistors which drive the LED's (you'd need one more transistor per digit, plus a couple resistors to limit the base-emitter current and collector current of the new transistors).

Yes, the digits would cause a voltage drop, and in the "off" state, they shouldn't be flowing any current because there should not be enough voltage difference to forward bias them.  However, the transistors would ALWAYS be forward biased due to the ~ 2 -3 V difference between the higher voltage at the emitters (~8V) and the bases (~5V).  That's an emitter-base current of ~2mA flowing into the microcontroller pins, even when you intend the transistors to be "off", with a 5V logic HI at the base.

Additionally, you'd need to check the current requirements of the new LED's as you'll likely have to make a change to the current limiting resistors R9 through R15.  If you are going to be sending more current through the transistors, you'll want to check to make sure you won't be exceeding the power dissipation limits.

I'm sure there's more but that's all I can think of at the moment.

I came across this tread; I wonder if it works ?

I do not want to end up buying one chip for each digit; that is not feasible; even if cheap chips found, the time to solder them is costly

http://archive.siliconchip.com.au/cms/A_112106/article.html

The way I see it, you need to add 14 devices. One to each of the pins that connects to the 7 seg LED and the LEDs in between.

Yes you may require to change the series resistors to suit the current.

I would suggest trying Opto Couplers, since there is no logic conversion, and they will handle the 7.2v without any issues.

I don't know what (if any) the potential for damage is here to your micro-controller

The general rule for voltages is NEVER apply a voltage to the IC pin that exceeds the voltage supply to the IC (unless specially designed for it)

Mark

Yes most use some form of FET or what ever in the bottom of each leg but as you say that would invert the logic required to set the cathodes to a particular logic level.... All personal choice

Yes most use some form of FET or what ever in the bottom of each leg but as you say that would invert the logic required to set the cathodes to a particular logic level.... All personal choice