74ls273

[Akoutsakas]

hello! i have a project in witch i want to use 8 74ls273 that drive 8 seven segment leds.
74ls273 has 20 pins. 8 input 8 output, vcc, gnd, MR (that i use always in High state) and Clk.

At the moment i want to test just one led, so i have connected only one input/output (the dot of the seven segment).
i get power from a transformer(220 to 4,5v) and i have put 2 buttons, one for the clk that is connected to High so when i push it goes to Low. and the one input to Low. so when i press it shall send input to 74ls273 that drives to led.
That doesn't work...the dot lights at completely random....Any ideas? anyone has used 74ls273 before?
an example of 74ls273 is here "http:hobby_elec.piclist.com/e_pic6_a4b3.htm"
thanx

 

[davenn]

hi,

did you wire the single LED in the correct way ie. did you use the resistor between the output ofthe LS273 and the transistor and the resistor between the transistor and the LED ?

if not maybe that why the cct is not controlable

try that and tell us the results

Dave

 

[Akoutsakas]

i have used one resistor just in the input of the seven segment.what u mean use resistor " between the output ofthe LS273 and the transistor"???
thanx for the reply.

also shall i use all the inputs and outputs so that ls273 shall work properly? or it doesn't matter and just one input output is enough?

 

[davenn]

i have used one resistor just in the input of the seven segment.what u mean use resistor " between the output ofthe LS273 and the transistor"???
thanx for the reply.

did you wire the 74LS273 as shown in the diagram ? a resistor on the 'Q' output to the transistor and through another resistor to the single LED segment. ?

You need to do that as the LS273 cannot drive the LED directly. Thats why the transistor is in circuit. The LS273 output turns the transistor on via the resistor in the 'base' leg of the transistor. The turned on transistor then allows current to flow from the +12V line through the LED segment through the resistor between the LED and the transistor 'collector' leg down through the emitter to gnd.

Dave

 

[Akoutsakas]

no i didn't wire as shown in diagram.actually i don't use transistors at my circuit. i just connect output to the seven segment.
i power up everithing with 5V, and i only use one resistor for the seven segment.
does ls273 need transistors to work? and if so what kind shall i use?

 

[davenn]

no i didn't wire as shown in diagram.actually i don't use transistors at my circuit. i just connect output to the seven segment.
i power up everithing with 5V, and i only use one resistor for the seven segment.
does ls273 need transistors to work? and if so what kind shall i use?

as I said in the last post yes it does and for the reasons stated

first thing you need to learn is to follow a plan. if the circuit diag has components in it you have to use them else it wont work properly the circuit isnt drawn that way just for fun

ok the resistor between the output of the LS273 and the base of the transistor could be ~ 1.5k (1500 Ohms) the transistor could be any general purpose NPN one like a BC546, PN2222, 2N3904 etc. the text for the circuit stated that the resistor between the transistor's collector and the LED in the seven segment display should be ~ 100 Ohms for a ~ 22mA current through the LED from a 12V supply.

speaking of power supplies, note that you need 2 different supplies.....
a 5V one for the LS273 and a 12V one for the displays

see how you go and report back

cheers
Dave

 

[Akoutsakas]

i made it work finaly, but when i change the power voltage it somehow reverses the input/output result. soon i ll post my circuit diagram. any idea about the power voltage of the clock? what's the ideal voltage to work?

 

[(*steve*)]

74LS will operate from 4.5V to 5.5V. Don't use them outside that range.

If you're using the outputs to switch something running from a higher voltage then the output swing of the LS series TTL may be insufficient with a higher supply voltage.

 

[davenn]

74LS will operate from 4.5V to 5.5V. Don't use them outside that range.

If you're using the outputs to switch something running from a higher voltage then the output swing of the LS series TTL may be insufficient with a higher supply voltage.

Thanks Steve
I was out of commission for several days and didnt get a chance to reply to Akoutsakas

yes exactly on the switching..... hence the use of a transistor between the LS237 and the
seven segment diplay. For what ever reason the displays in the cct him posts links to were running from a +12V rail, maybe just easier to use what's readily available

ciao
Dave

 

[Akoutsakas]

Well i made it work finaly (at least for a while). i found a 5v power supply and it worked fine. then i connected my circuit to 8088 and it din't work as i waiting. anyway. i want to find if my 273 ar still working fine or they r destroyed (cause i can't find any new now).
i connect them to my breadboard but i can't make them work. i used some new 374 that ar almost the same and they don't work also...so i guess i do something wrong.... j1 is the input. CP is also connected to input. Any idea how i can give a proper CP? till now i had CP connected to + or - and it work as soon i was disconected it. thanx!

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