LED-Display problems

[Lac]

I'm about building a Digital watch using the 7447 ICs and some common led-displays with a dot (D/P). But then I saw that the 7447 don't have a D/P pin for the dot on the led-display, at least as far as I can see.

How can I solve this out? just connect 5v to the D/P pin on the led-display?
But what if I want it to blink once a second?

Cheers!
Lac.

 

[Ravi]

Lac,

I do not have much experience on watch projects.But some time ago I used 7447 to drive a 7 seg.LED display.You are right Lac. 7447 does not have a D/p pin. I assume that your problem is how to blank off leading and/or trailing zeros. right? You can do it by using a ripple blanking rechnique. The 7447 has ripple blanking output (RBO) and input (RBI) terminals. Thay are pin 4 & 5 respectively. If the RBI terminal is held low,(@ logic-0) the seg. outputs of the IC are enabled but the RBO terminal is disabled (held low). If the RBI terminal is biased high (@logic-1), the outputs become disabled in the presence of a BCD 0000 input (=decimal zero) and the RBO output goes high under the same condition.Therefore, the RBO terminal is normally low and goes high only if a BCD 0000 input is present at the same time as the RBI terminal is high.

I assume that your watch has 4 separate 7 seg. displays with respective decoder/driver 7447 ICs.I have drawn a block diagram which shows the ripple blanking technique is being employed to provide leading zero suppression. Here, the RBI input of the 1000's or MSD IC is tied high, so this automatically blanked in the presence of a zero, under which condition the RBO terminal is high.Consequently, the RBI terminal of the 100's IC is high and the RBO terminal is low.The RBI input of the 10's unit is thus also low and its RBO output is low.The LSD unit does not require zero suppression,so its RBI input is grounded.Your must be knowing that 7447 has an active low output designed for driving a common anode LED display via external current limiting resistors.

 

[Sebi]

The decimal point in LED-display only a LED. If the display common anode type, just connect the DP pin to GND with serial 470ohm resistor for activating.