Classic LED 7-Segment Displays

Just recently I have been addicted to old LED displays as they are small and bright and I love the classic look. We can see them in vintage calculators and vintage led watches. However these displays consume significant amount of power, so they are not used in watches and calculators anymore. As they are replaced by LCD, these LED 7-Segment displays are not in production anymore and difficult to obtain.

Now, I have 2 models of the classic LED 7-Segment as shown in the picture below: HP 5082-7414 from HP is on the left. It’s a 4-digit Red LED 7-Segment very nice for wristwatch. The one on the right is an 2-digit Red LED 7-Segment from an unknown maker. It can be used in a wristwatch too (with a little bit bigger case).

Based on my inspection, I have made symbols for these displays with Eagle 5.4.0 free version. The displays are common cathode and the symbols are below

The PCB footprints are as the following (DIP 12)

I have made simple clocks using these displays and PIC16F887. The real thing looks much better than the photo. The displays are bright red and sun light viewable. Very COOL!!! They are on my computer desk and I love to see them very often.

Each clock consumes about 0.25W (50mA, 5V) when the PIC16F887 operates at 250kHz (display refresh rate is about 61Hz). The amount of consumed current can be reduced significantly if I use some current limit resistors. But the displays will be dimmer than without resistors. I will try to use PWM for reducing the power consumption as I don’t want to put 8 resistors into my design. The schematic/PCB and firmware including source code in MikroC will be made public once I have complete all of the designs. I will even have kits for sell if my time permitted.

1Hz Clock Generator using PIC12F675

Based on the idea from http://www.josepino.com/pic_projects/?timebaseI have created a 1Hz Clock Generator. I use PIC12F675 as it's available locally. Its price is just about US$1.
The concept is using 32.768kHz crystal as a clock for the PIC. Therefor, the internal instruction clock is 32768/4 = 8192 Hz. By using the 16 bit Timer1 to count the instruction clock cycles, the interrupt will occur every 8 second. This period can be reduced by setting initial value of the Timer1 (TMR1H:TMR1L). I have to make Timer1 to count up to 8192 for generating overflow interrupt every 1 second. To make Timer1 count up to 8192, the initial value of TMR1 must be 65536-8192 = 57344 or 0xE000. This means TMR1H = 0xE0 and TMR1L = 0x00. In this case, I need to set only the TMR1H=0xE0 and let TMR1L runs continuously. By changing the initial value of Timer1, I can generate almost any frequencies.

An application for this project is a precise 1Hz blinking LED signal :) ha ha. I know that it's not useful but I think it's fun to look at (am I crazy?). Another application is a precise 1Hz time base for a clock.

The source code is written in MikroC.

// PIC12F675
// 1Hz Time Base Osc.
// Timer1 Module
// 32.768 KHz
unsigned short tick;
void Init ();
void interrupt ()
{
if (PIR1.TMR1IF)
{
TMR1H = 0xE0;
PIR1.TMR1IF = 0;
tick = 1;
}
}
void main ()
{
tick = 0;
//Initialize Ports and Timer1 Module
Init ();
while (1)
{
if (tick)
{
tick = 0;
GPIO = (1 << 2);
}
if (TMR1H > 0xF0)
{
GPIO = 0;
}
}
}
void Init ()
{
TRISIO = 0;
//Make all pins as output ports
GPIO = 0;
//Use Timer1 module
INTCON.GIE = 1;
INTCON.PEIE = 1;
T1CON = 0x01;
//Overflow every 8192
TMR1H = 0xE0;
TMR1L = 0x00;
// Enable TMR1 interrupt
PIE1.TMR1IE = 1;
}

The schematic is as the following image.





The PCB:



3D version: