Pulse counter using Digital IC and 7-seg display.

Thread Starter

yesilovethis

Joined Jul 28, 2016
23
Hi All,
Before I apply the circuit you have suggested, I encountered a strange situation with 7segment display. The driver IC is 74ls49 (open collector). From my lab I got common anode 7segment display. So on a breadboard I wired the 74LS49 and that 7seg display as following:
Anode of 7seg display to +5V, and the a to g pins of 7seg connected to a to g pins of 74LS49 with 510ohm registors (this limits the current to 5mA for each segment of the 7seg). But here comes the funny part, with all input A,B,C,D of 74LS49 to ground (BCD zero) I see number 8 in the 7 segment!! So my logic got inverted. With 0001 BCD I see mirrored 3 in the 7seg. I am going to check if the lab has common cathode 7seg, that will make life easy. But if there is not, would it be possible to use common anode anyway?
 

eetech00

Joined Jun 8, 2013
3,857
Hi

The 74LS49 has open collector outputs, so If you use common anode LED display, pull-up each 74LS49 output to 5v.
All LEDs will then be off. When the output turns on and switches to ground, the LED will turn on thru its current limit resistor.
See example attached.
 

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Bernard

Joined Aug 7, 2008
5,784
Same error as in schematic of counter.
Only thing that I can think of is use an invertor like ULN2003, 7 darlingtons,BCD Counter#2 00000.jpg or use a common cathode display as in thumbnail. R TBD depends on display color; if red, 330 might be about right.
 
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AlbertHall

Joined Jun 4, 2014
12,344
Same error as in schematic of counter.
Only thing that I can think of is use an invertor like ULN2003, 7 darlingtons,View attachment 111895 or use a common cathode display as in thumbnail. R TBD depends on display color; if red, 330 might be about right.
From the datasheet, for an input of zero, all segment outputs are high except segment g which is the bar across the middle. For this to display zero, when a segment output is high that segment should be lit. You can't do this with a common anode display unless you put something inverting in between (ULN2003?). The '49 can only directly drive common cathode displays as in Bernard's picture. In that arrangement I think the TBD resistors should be zero Ohms i.e. they are not needed. The current through the '49 outputs will be limited by the LED current limiting resistors.
 

Bernard

Joined Aug 7, 2008
5,784
The output sink ( bring low ) current is rated at between 6 mA to 8 mA; Vcc = 5 V, sat. V .4 V, load = 510 ohms
I out then 9 mA. By adding 240 ohms between output & LED anode, output current drops to about 6 mA, LED V about 1.5 V, not enough to turn on. That is if a common cathode display is available.
 

Thread Starter

yesilovethis

Joined Jul 28, 2016
23
Hi All,
No worries with display, I bought 7seg display with common cathode. Now it work as expected as you can see in the picture. On left of breadboard is lm311 which I was checking earlier to see minimum input voltage to change the output. Now I need to wire up (probably solder on a pcb) the comparator, the bcd counter and the 74LS49 with the 7seg. Lots of soldering though, I have to figure out which board is suitable the parallel or the dotted.
 

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AlbertHall

Joined Jun 4, 2014
12,344
The parallel needs planning to be able to use the tracks effectively and for this kind of circuit it would likely be easier to use the individual pad king of board then you can put the ICs where you want and wire as appropriate.
 

jkwilborn

Joined Jan 26, 2016
32
I used to build stuff, tubes, then transistors, then IC's, 8008 and so forth. Now I keep a bunch of atmega328p micro controllers on stock and then program them to do most things that I used to have to piece together, with sometimes no more than a since micro. It can count from an external source or give you times between pulses, if you want to do such things. Unimaginable flexibility from an I/O perspective and multiple timers, great fun. I do understand using the older stuff, but it's much more complex, I think. I use the seven segment devices also, and just set up a common bus, put the data on the buss, then pull each of the devices cathode to ground sequentially (after changing data on bus) with a mosfet. I use three displays and give them about 10 ms each and get a good display. Lots less wiring... I also program the micro to use the seven segments to display 0-f (nibble), which takes a different chip, but the micro can do both :)

This is a great idea and an interesting application. Just needs an external 'flash' and sound detector and you could tell how far away it is.
 

ian field

Joined Oct 27, 2012
6,536
Hi all,

As a part of Hobby I made a VLF receiver which can detect close and distant thunder strikes (which are broadband spikes in the waterfall display). I want to extend that project by adding a counter to count those spikes per second (which usually range from 1 to 10). This would need usage of digital ICs, BCD to 7seg IC which unfortunately I don't remember now (I learned it during my masters but forgot due to no use in my current career).
The scheme is straight forward and seems do-able. I have attached the block diagram (the wiring is abstract as I don't remember the ICs). Only first stage is done (the receiver). The next step (starting from clipper) I am having trouble with to start. Please have a look and advice me how can make it, and what ICs arView attachment 111606 e needed. I want to count pulse per second, not continuous. If you have/propose a schematic/circuit for this project, please let me see.
Many thanks.
You need a couple of clocks - one to reset the counter every second, and another to latch the count into the D-type flip-flops that appear to be missing from your schematic.

The timing generator can be pretty crude if you run the output pulses through a CR differentiator - that produces a narrow spike just enough to drive a reset or latch input, but not wide enough to make a significant dent in the timing period.

Donald Lancaster published TTL and CMOS Cookbooks - the technique is described in one or maybe both. They exist online as PDF, but take some finding.
 

ian field

Joined Oct 27, 2012
6,536
The LS49 was available, common cathode now in hand.
I have a vague recollection of a chip in the 4000 CMOS family that combines the decoder-driver and latch in the same chip. Not sure if there's one with the decade counter as well - probably not.

Or I might have that the wrong way round - maybe a counter/latch to use with the 4511.
 
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