How to read a datasheet (74LS04)

[mdfrahim1]

Dear All,
I am an Electronics engg. by qualification but am working in software industry for nearly 10 years. Never did anything on electronics. But now i want to start building small projects, just out of curiosity.

To start with I brushed up my digital theory concepts and tried to start building small circuits using IC's. But while using IC's I am facing few problems.
1. Although the data sheets are available I am not able to interpret the information fully.
2. It never works as specified :-(
3. I am trying to use 74LS04 (NOT GATE) just to see if it works or not. Please let me know how to make it work. Here is what I did.
Used a Breadboard , connected the IC with a 4.5V supply (Vcc) at pin 14 and connected the pin 7 to ground.
Q1. After doing this what will be the voltage and current available at the remaining pins?
Q2. I measured the voltage between pin 14 and ground and it was about 2 volts!!!! am not sure where the remaining 2.5V went ?
Q3. I measure the voltage at the input pins and it was about 1 .1 V and at the output pins it was about .2V. Is this correct ?
Q4. I gounded input pin 1 and the output pin 2 showed me a voltage of .9V. Is it correct ? I mean is the IC supposed to do this ?
Q5. data sheet says that Output high voltage (Voh = 2.7V). So when i grounded pin 1 (means Input Low Voltage,Vil=0) should,nt i recieve 2.7V at pin 2?

Can someone please explain how do we practicall operate an IC. I just need that if I am giving a current to Pin 1 then Pin 2 should not give any current. How much current these pins conduct ? Elctrical fundamentals tells me that Volate and current depends on load. So how the datasheet specifies current and volatage without knowing the load !!!!!!!!!!!!!!!

Please help me get started with this. Thanks in advace.
Moderator,
Will it be possible to make a sticky on the basics of reading a datasheet and interpreting the information correctly. I think it will help a lot of people like me.

 

[ericgibbs]

hi,
If you have connected a 4.5V battery, +4.5v to pin 14 and the other side of the battery to pin 7 and you measure only about 2V, then either the battery is no good .[discharged] or the 74LS04 is faulty.

You should read +4.5v between pins 14 and 7.

The other voltages you have measured will not give correct readings for the 74LS04 with such a slow supply voltage, of 2v.

The datasheet voltages, Vhi, Vlo, are given for a supply voltage of +5v.

 

[mdfrahim1]

Hi Eric,

I had tried this for 4 IC's but all of these shows a voltage drop. How should I check if the Voltage supply is correct or not ? When I measue it without an IC , i get a correct reading !!!!! As soon as I connect the IC the reading drops. Please help me....

 

[ericgibbs]

Hi Eric,

I had tried this for 4 IC's but all of these shows a voltage drop. How should I check if the Voltage supply is correct or not ? When I measue it without an IC , i get a correct reading !!!!! As soon as I connect the IC the reading drops. Please help me....

hi,
What is the actual battery voltage when you have a 74LS04 connected to it??

In other words, whats the battery voltage when its on load: connected to the ic??

Measure at the battery terminals and also at pins 14 to 7 on the ic. Tell me waht you read.

 

[Dean Huster]

Be sure that you have the positive end of the battery connected to pin 14 and the negative end to pin 7. Reversing this polarity will destroy (or at least compromise the specifications of) the IC. Reversal will also probably draw a lot more current which the battery cannot handle and the voltage will drop.

When power is properly established, right polarity, right voltage, good battery, then the inputs must be considered. A "floating input" (one that is left unconnected) will usually "float" to a logical HIGH (+Vcc) and the output of that particular inverter (NOT gate) will go to a logical LOW (gnd). Inputs must be committed to either a HIGH or a LOW.

Dean

 

[k7elp60]

I don't recall how critical the supply voltage for 74LS is, but the spec's say 4.75 to 5.25V. Is it possible the supply voltage 4.5V is too low?

 

[audioguru]

The 4.5V supply drops to 2V when the IC is connected because the 4.5V battery is dead. It died when it saw the awful old TTL IC coming.

TTL draws too much current to be powered from a battery. Cmos should be used instead.

 

[saturn1bguy]

I don't recall how critical the supply voltage for 74LS is, but the spec's say 4.75 to 5.25V. Is it possible the supply voltage 4.5V is too low?

Nope, that's not a problem. The part may not work correctly with 4.5V, but that level won't cause the supply voltage to sag to the 2.0V he mentions.

(Did the follow ever say he was using a battery? I just simulated his symptoms with my power supply. I was able to adjust the current such that the voltage was 4.5V/2.0V before/after connecting the part, with the supply going into constant current mode. Perhaps he's just unlucky enough to have the current knob in a bad place...)

Mdfrahim1, if you're using a bench power supply, crank the current up a bit; if it's a battery, charge it up.

Just a thought,
Corey

 

[matc]

Are you really an electrical engineer?

TTL will work fine at 4.5v, it just won't operate up to spec in terms of voltage levels and speed.

Joking aside, batteries will power TTL logic just fine - but don't use it in something you hope to operate for months at a time. Quiescent current is something like 3 to 6mA. That's something of the order of 800 hours for AA batteries.

I suspect you're not finding pin 1 correctly, or your voltmeter is seriously flawed. Unless the batteries are totally wiped out, you'll get almost no voltage drop when you power up a 74 series chip, so you need to make sure the power is correct before you hope to get things working.

After that you'll see the inputs float high and the outputs get driven low, and your problems reading the datasheet will disappear. I cut my teeth on 74LS circuits, never had them not work as specified.

 

[mdfrahim1]

I am sorry , I should have mentioned the complete scenario.

1. I am not using a battery but a transformer which is converting from 220V AC to 15V AC. Then I am using a rectifier(combination of 4 doides and a capacitor) to convert it to 15V DC. Here I had used a capacitor of 1000 micro farad (and I have no idea Why ? I just saw someone else using it so I also used it !!!!!! Can someone pls. explain how to determine the value of the capacitor to be used?)

2. Then I tired to convert the 15V DC to 4.5 V DC using some resistors. I used 3 capacitors of different values and got 4.5V across the 3rd resistor. Then I connected the +ve of the resistor to pin 14 and -ve of the resitor to Pin 7.

3. I am an electronis engg. but did my degree 10 yrs. back and since then I am in the Software field. I am a consultant on AS400 and a Retail domain expert. For few months I am trying to build a circuit for automatically swicth off/on the motor whcih fills my water tanks. I do google to get this circuit but never understands it so I thought of going with my own logic. I have 3 tanks on my roof. I am planning to put 1 float switch in each of the tank and 1 float switch to the water input supply. When water comes I will get a signal (means some current will flow) and when Tank 1 is full I will get a signal (as the float will complete the ciruit) similarly for tank2 and tank 3.
Now say Water inlet means A, and tank 1 means T1,tank2 means T2 and tank3 means T3.

To start the motor - I need to A - High and either of T1,T2,T3 to be low (means either of the tank not fill).

To switch off the motor - I need A - High and all of T1,T2 and T3 to be High.

I am thinking to use a AND gate to achieve this combination but in that I was getting some problem so I thought of invetring T1,T2 and T3 before feeding it to the IC for AND Gate. The output of the AND IC will be used to drive a Relay.

I will wind up now before this becomes too confusing.

Will highly appreciate all of you if you guys can help me make this circuit. I want to make it by my own no copying. This is the way we learn.

Just let me know which IC shall I use for OR and AND gates.

 

[ericgibbs]

hi
>> 2. Then I tired to convert the 15V DC to 4.5 V DC using some resistors. I used 3 capacitors of different values and got 4.5V across the 3rd resistor. Then I connected the +ve of the resistor to pin 14 and -ve of the resitor to Pin 7.

You cannot get a 'fixed' 4.5V by using a resistor/capacitor network in this way.
As the current drawn by the 74LS04 changes so will the voltage drop in the resistor string and the voltage on pin14 will change.
This is basic Ohms law!.

Get a 7805 +5V regulator, they are quite cheap and readily available.

Connect the input of the 7805 to the +15V supply, place a 47uF cap and a 0.1uF cap across the +5V output to 0v,
this will solve the voltage supply problem.

 

[mdfrahim1]

Thanks a lot Eric.

Now 7805 voltage regulator means LM78M05C or what ? Can I use LM341. It also shows the same characteristics. I googled on the data sheet but am only to fine that 7805 works on an input voltage of 6 to 12V ? Is it true ? if it is a regulator then why its dependent on input voltage (ofcourse it shoyld be more the 5).

Q2. Why are we using capacitor ? Please explain. The use of capacitors really haunts me. I will be highly thankfull to you if you explain me.

Looks to me that I will be able to complete my circuit now.

 

[ericgibbs]

Thanks a lot Eric.

Now 7805 voltage regulator means LM78M05C or what ? Can I use LM341. It also shows the same characteristics. I googled on the data sheet but am only to fine that 7805 works on an input voltage of 6 to 12V ? Is it true ? if it is a regulator then why its dependent on input voltage (ofcourse it shoyld be more the 5).

Q2. Why are we using capacitor ? Please explain. The use of capacitors really haunts me. I will be highly thankfull to you if you explain me.

Looks to me that I will be able to complete my circuit now.

hi,
The 7805 will work upto about 30Vin.
If the LM341 has the same spec as the 7805, use it.

A standard regulator requires about 3 volts more on its input than its output voltage, else cannot regulate.

The capacitors are required for stabilty and also act as smoothing.
Your circuits needs them, so fit them.

 

[mdfrahim1]

The capacitors are required for stabilty and also act as smoothing.
Your circuits needs them, so fit them.

How do you arrive at 47uF cap and a 0.1uF ????

I need to know so that next time I can calculate these myself,next time

 

[shaneshane1]

How do you arrive at 47uF cap and a 0.1uF ????

I need to know so that next time I can calculate these myself,next time

personally im not sure how this is worked out, but the whole idea behind using the caps is to smooth out voltage so you get an even supply(no spikes) so i would assum that as long as the cap can fill up really fast and stay full while the circuit is running then that should be sufficiant.

thats what iv always done and i dont have any problems

thats just my opinion,But others may give better description

 

[ericgibbs]

personally im not sure how this is worked out, but the whole idea behind using the caps is to smooth out voltage so you get an even supply(no spikes) so i would assum that as long as the cap can fill up really fast and stay full while the circuit is running then that should be sufficiant.

thats what iv always done and i dont have any problems

thats just my opinion,But others may give better description

hi,
The 0.1uF is a requirement from the regs datasheet, without it, the reg can be unstable and oscillate.

The larger capacitor is there to provide a current 'reservoir', should the load the reg is powering, draws a heavy transient current.

Some regs are unable to respond fast enough to supply a transient heavy current
and the regulated output voltage would fall during this transient.

This transient fall in voltage could be seen by the other parts of the load circuit and cause mis operation.

The smaller 0.1uF also has a much lower impedance to transients that of a electrolytic cap and can effectively bypass 'spikes' to 0V.

Do you follow this?.

EDIT: Some bed time reading...

**broken link removed**
http://www.kpsec.freeuk.com/powersup.htm
**broken link removed**

 

[matc]

You can calculate the cap values you need from the current draw (not much until you hang relays and motors off the same power supply), input frequency (100 Hz for a full rectifier), and allowable droop between input current peaks. But for now, those values will be fine.

The problems you saw were because voltage dividers make a very poor power supply. V = I * R, so the voltage drop across each resistor depends completely on the current flowing through it. Any voltage reg sheet will do far better than this - they include a voltage reference and amplifier rather than a divider.

Consider not using any logic at all if things for your tanks are as simple as you outline. You can either put switches in parallel (to OR the signals together), or in series (to AND the signals), or you can use diodes into relays (generally this will OR signals together, but if you use the normally-closed relay contacts you get a NAND circuit).

The advantages of all these techniques are that they will operate on a large range of input voltages and are very robust.

 

[mdfrahim1]

Thanks eric and Matc for your replies.

This is what I want.

Input T1 (Tank 1) T2 (Tank 2) T3 (Tank3
(Incoming water)
1 0 0 0 Run motor
1 0 0 1 Run motor
1 0 1 0 Run motor
1 0 1 1 Run motor
1 1 0 0 Run motor
1 1 0 1 Run motor
1 1 1 0 Run motor
1 1 1 1 Shut down motor

Note: Tanks - 0 means Tank is empty
Input (Incoming water) is 1, if water is coming.

When Incoming Water is 0 then I will not run the motor irrespective of tanks.

Can someone explain how shall Igo ahead with this ? I was using IC because this way 1). I will build the circuit 2). Learn how to use an IC.

The above logic does not have any direct Gate ? I will have to somehow simulate it.....

If you guys suggest me to do this without IC, please explain how ?

 

[matc]

So you have this:
10xx = 1
1x0x = 1
1xx0 = 1
0xxx = 0

Where X means "don't care".

Assuming you have microswitches to do your tank sensing:
Wire your tank switches in parallel, but use the "normally closed" part of the microswitch.
Wire that in series with your incoming water sensor (normally open).

So you'll only have the motor running when the incoming water sensor is at 1.
And only if at least one of the tanks is empty.

 

[crashsite]

I am not using a battery but a transformer which is converting from 220V AC to 15V AC. Then I am using a rectifier(combination of 4 doides and a capacitor) to convert it to 15V DC. Here I had used a capacitor of 1000 micro farad ...

All sounds okay to that point. If you are in an experimenting mood, get a 7805 regulaor IC. You can probably find one on an old circuit board as they are extremely common. This sill convert the rectifier/filter output to 5 volts which is just what you need for TTL (Like the 7404).

Here's a note about TTL logic. You can't just leave input pins unconnected. If you do, internal resistors will pull the input to a logic high.

I still use TTL sometimes because it's cheap and rugged (not to mention that I have a ton of it in my junk drawers). Besides, there's a certain satisfaction in working out the logic and seeing it work on a breadboard that I think software types must miss out on.

But, it's true that it's no good for battery operated stuff because it does draw a lot of current.