(A) This looks like a good time to make the jump to surface mount. Try it, you'll like it !!!.
(B) Display driver alternatives.
35 LEDs driven from a single package, driven by a single output pin.
If you want to spoil all the fun and get a compact solution that works (but may not challenge your board layout skills so much, consider solutions such as:
MM5450 35 LED driver. Datasheet This is my favorite from way way way back. Once you use one of these you'll be spoiled. 35 LEDs from one output pin ! :-). In terms of cost effectiveness and simplicity of drive little else compares. In stock at Digikey for $US4.39/1 in DIP40, and also in stock in PLCC 44 $3.78/1. Chainable with a little work. Notionally requires 3 lines to control it but the excessively enthused can do it with 1 line and a few RC delays. It works :-). They say: Data is transferred serially via 2 signals; clock and serial
data. Data transfer without the added inconvenience of
an external load signal is accomplished by using a
format of a leading “1”followed by the allowed 35 data
bits. These 35 data bits are latched after the 36th has
been transferred. This scheme provides non multiplexed,
direct drive to the LED display. Characters currently
displayed (thus, data output) changes only if the serial
data bits differ from those previously transferred. Note the cut and paste typo on page 5 of the data sheet. How to drive with one output pin - see at end.
TLC59282 16-Channel, Constant-Current LED Driver Texas Instruments' TLC59282 is a 16-channel, constant-current sink driver. Datasheet here. Each channel can be individually controlled via a simple serial communications protocol that is compatible with 3.3 V or 5 V CMOS logic levels, depending on the operating VCC. Once the serial data buffer is loaded, a rising edge on LATCH transfers the data to the LEDx outputs. The BLANK pin can be used to turn off all OUTn outputs during power-on and output data latching to prevent unwanted image displays during these times. The constant-current value of all 16 channels is set by a single external resistor. Multiple TLC59282s can be cascaded together to control additional LEDs from the same processor.
TLC59281 . Similar.
- Allegro 6276 Suspiciously similar functionally.
The A6276EA and A6276ELW are specifically designed for LEDdisplay applications. Each BiCMOS device includes a 16-bit CMOS
shift register, accompanying data latches, and 16 npn constant-current
sink drivers. The CMOS shift register and latches allow direct interfacing with
microprocessor-based systems. With a 5 V logic supply, typical serial
data-input rates are up to 20 MHz. The LED drive current is determined by the user’s selection of a single resistor. A CMOS serial data
output permits cascade connections in applications requiring additional
drive lines. For inter-digit blanking, all output drivers can be disabled
with an ENABLE input high. Similar 8-bit devices are available as the
A6275EA and A6275ELW
One pin drive
Some IC's with clock / latch / data / reset control lines often seem to have been obligingly arranged so that the polarity of the control signals allows time delays and a single input line to control the IC. Worst case with less obliging IC's a few inverters may be needed.
Clocking on an MM5450 occurs on the rising edge. Data is fed from the input line via an RC delay, with t delay several times greater than the achievable clocking rate. If the input line is held high for Td data goes high. If input is now lowered and raised a high is clocked in. If, instead, the clock line is lowered as soon as possible after the last clock edge and held low for Td and then raised a low is clocked in. A reset or frame load or whatever signal can be achieved by a longer delay of say Treset. Holding the input at the reset level (whether high or low) for > Treset will activate the reset or frame load or whatever function. Reset delay can be restarted by using a diode across the charging resistor so that reseting action is reinitiated every time an o[[osite condition appears on the input line. If copious processor pins are available and the distance to the display IC is small then using multiple drive lines may be preferred. However the above scheme can be used to operate a display IC via a single pair at a distance. For extra points the same pair can be used for power supply. Powering and full control of 35 LEDs on one pair is a good trick if you can do it. With an MM5450 you can. Long ago this was an attractive solution in some cases. Now, in the age of 50 cent (or less) microcontrollers, placing a controller at the "far end" may be preferred.
Brighhtness & current:
LEDS get approximately linearly brighter with increasing current. A current of 15 mA per segment is quoted (which is a minimum value, max being 25 mA+). Many modern LEDs are specified at 20 MA max continuous. A ratio of 3:4 (= 15mA : 20 mA) is virtually indiscernible to the eye, and the efficiency of the LEDs is a far larger factor than small changes in current.
At say 10 mA there are many modern red LEDs that would be as bright as you'd want - and far brighter than LEDs from previous generations. Look at what people like Nichia & Cree are offering at the top end and you'll be duly impressed.
That said, the IC is limited in output if you need all 35 segments on at once. With 15 segments lit you can get 20+ mA (fig 7).