Hi!
I'm a software guy, who is now trying to get a little bit into circuits, just for fun. I.e.: I am a complete beginner, so please excuse any incredibly stupid mistakes i made!
Now that that's cleared up:
I am trying to manually light up some LEDs with the 74LS164 8bit shift register.
You can see a photo of my circuit (which I hacked together according to various pretty pictures I found online) here:
http://gometa.org/shift_register.jpeg
The idea is that the left push-button is supposed to set the input pins (1 & 2) to 5V, and the right push-button is supposed to set CLOCK pin to 5V, that the the...erm...registers may shift. The RESET pin is, right now, wired to Vcc via a 4.7kohm resistor (although i also tried it without the resistor)
The ULN2803 in the upper left corner is supposed to drive some bright LEDs someday, and isn't causing any discernible problems right now, so it may be ignored.
I am using the 5V (up to 2Amps) cables of an old external hard drive power supply to power this thing.
My problem is that nothing seems to work, and, even worse: very strange things are happening.
a) the buttons don't seem to do anything. The voltage on pins 1&2 or the CLOCK pin goes to 5V and all, but the output pins happily ignore that fact.
b) I noticed (on two separate ICs) that there is aprox. 1.2V between pins 1&2 and GND, even if I only connect Vcc and GND to the IC. I read somewhere that the IC only regards < 0.8V as 'off', so this might be a problem. Where this voltage is coming from, I don't know.
c) If I touch (!!!!) the CLOCK pin with my multimeter, all the outputs go up to 5V, and the connected LED merrily blinds me. If I touch the RESET pin, the outputs go down again. This intrigued me, so now i stumbled across the REALLY weird thing: if I touch those pins with an insulated wire, that is not connected to anything (!!) - not even my hand or GND - the same thing happens. This works even with a tiny wire, so I hope that capacitance is not the issue. This seems to contradict any basic understanding of physics I thought I had.
I googled around, and most of the internet seems to agree that
a) pull-up and pull-down resistors are nice and all, but shouldn't really be an absolute necessity for a simple thing like this (and, I don't quite 'grok' them)
b) I was afraid that I somehow fried my ICs, but noone else seems to have a problem connecting them directly to their power supply.
So, could anybody please give me a hint what the heck I am doing wrong? And why in the name of everything that is holy (and unholy) does the single end of an otherwise unconnected wire to the CLOCK and RESET pins make the IC do stuff (even if it's the wrong stuff), especially if blasting them with 5V doesn't seem to do anything at all?
Thanks, and sorry that this post got rather lengthy.
-M.
P.S.: Datasheet for the 74LS164: **broken link removed**
P.P.S.: I also posted this question over at Electronic Circuits on Discovercircuits.com - Free Electronic Circuits - I'm sorry about that, but I am desperate. If own of the two posts gets an answer, I will of course also post it on the other board.
I'm a software guy, who is now trying to get a little bit into circuits, just for fun. I.e.: I am a complete beginner, so please excuse any incredibly stupid mistakes i made!
Now that that's cleared up:
I am trying to manually light up some LEDs with the 74LS164 8bit shift register.
You can see a photo of my circuit (which I hacked together according to various pretty pictures I found online) here:
http://gometa.org/shift_register.jpeg
The idea is that the left push-button is supposed to set the input pins (1 & 2) to 5V, and the right push-button is supposed to set CLOCK pin to 5V, that the the...erm...registers may shift. The RESET pin is, right now, wired to Vcc via a 4.7kohm resistor (although i also tried it without the resistor)
The ULN2803 in the upper left corner is supposed to drive some bright LEDs someday, and isn't causing any discernible problems right now, so it may be ignored.
I am using the 5V (up to 2Amps) cables of an old external hard drive power supply to power this thing.
My problem is that nothing seems to work, and, even worse: very strange things are happening.
a) the buttons don't seem to do anything. The voltage on pins 1&2 or the CLOCK pin goes to 5V and all, but the output pins happily ignore that fact.
b) I noticed (on two separate ICs) that there is aprox. 1.2V between pins 1&2 and GND, even if I only connect Vcc and GND to the IC. I read somewhere that the IC only regards < 0.8V as 'off', so this might be a problem. Where this voltage is coming from, I don't know.
c) If I touch (!!!!) the CLOCK pin with my multimeter, all the outputs go up to 5V, and the connected LED merrily blinds me. If I touch the RESET pin, the outputs go down again. This intrigued me, so now i stumbled across the REALLY weird thing: if I touch those pins with an insulated wire, that is not connected to anything (!!) - not even my hand or GND - the same thing happens. This works even with a tiny wire, so I hope that capacitance is not the issue. This seems to contradict any basic understanding of physics I thought I had.
I googled around, and most of the internet seems to agree that
a) pull-up and pull-down resistors are nice and all, but shouldn't really be an absolute necessity for a simple thing like this (and, I don't quite 'grok' them)
b) I was afraid that I somehow fried my ICs, but noone else seems to have a problem connecting them directly to their power supply.
So, could anybody please give me a hint what the heck I am doing wrong? And why in the name of everything that is holy (and unholy) does the single end of an otherwise unconnected wire to the CLOCK and RESET pins make the IC do stuff (even if it's the wrong stuff), especially if blasting them with 5V doesn't seem to do anything at all?
Thanks, and sorry that this post got rather lengthy.
-M.
P.S.: Datasheet for the 74LS164: **broken link removed**
P.P.S.: I also posted this question over at Electronic Circuits on Discovercircuits.com - Free Electronic Circuits - I'm sorry about that, but I am desperate. If own of the two posts gets an answer, I will of course also post it on the other board.