I have spent so much time on this little circuit and yet it still doesnt work . Ok I have a LM35 (temperature sensor) . Its output needs to be amplified by 5. This then connects to a ADC (ADC0804). This value is then read by an 8031. The 8031 divides this value by 2.5 and outputs to the LCD. Ok so i know the LM35 outputs 10mv/C and thats why i need to amp it up a bit. so 1v = 100C (10mv * 100 = 1). Thats good enough for me as this device is not gonna go any where near 100C and will not go below 0. so 1v * 5 = 5v. So i need a gain of 5. Max needs to be 5v because I am using ADC0804 with a VREF of 5 or VREF/2 = 2.5 5/255 = 0.0196v per devision (255 = 2^8) ie temp of room = 21c LM35 ouput = 0.21V Amp output = 1.05V (= 0.21 * 5) ADC ouput = 53.57 (= 1.05 / 0.0196) 8031 ouput = 21.4 (= 53.57 / 2.5) temp of room = 30c LM35 ouput = 0.30V Amp output = 1.5V (= 0.30 * 5) ADC ouput = 76.5 (= 1.5 / 0.0196) 8031 ouput = 30.6 (= 76.5 / 2.5) Ok so thats the theory behind all of that yet it doesnt work. I know the fault is not with the 8031 or with the LM35 or with the ADC. The problem lies with the amp. I am using to a 741 to try * 5 but no matter what i do it just doesnt work. I have searched a bit on this form to try to find info on the 741 but just running into ppl saying dont use a 741 and i have come to the conclusion the 741 will not work for this. I only have 5v and ground to work with. If any 1 can help me please 2 things 1. What op-amp / IC must i use to achive this * 5? 2. What should my circuit look like? This is what i have been taught yet it doesnt work. By using this circuit i get a constant output voltage no matter what the LM35 changes too, the ouput of the 741 stays constant. It does change if i change the feedback var. Considering if i change the FeedBack var the lcd reflects the changes so i know the problem lies not with the ADC or 8031. If i stick the LM35 by itself and measure the voltage out it is correct and does change. I connect this all to the 741 and then nothing works like it should. Please if you can provide any help.
Narrow down the source of your problem. Apply a controlled voltage to the input of your amplifier and measure the output. Apply a controlled voltage to your A/D converter and check the result. Apply a known bit pattern to the register you are using to store your A/D result and check you get the correct output. http://ourworld.compuserve.com/homepages/Bill_Bowden/opamp.htm Shows how to connect an op-amp as a non-inverting amplifier.
I know its its the opamp that is the problem ... can this be done with a 741 ? from what i have seen it cant be. can any 1 provide the name of an opamp / compartator that will allow me to do this. From that link i will try that configuration for the non-inverting amplifier.
I think your problem is the 741 on a single-rail 5V supply. Most op-amp outputs can't swing any nearer than about 1.5V from either supply rail, so until your ingoing signal goes above about 0.3V it will give the same 1.5V output. An LM358 or LM324 is aimed at single-rail use, they will swing down to ground and do a better job in this situation. Your feedback resistors are wrong too, Rf should be 5k if Ri=1k
ok i built that circuit using a 741 if i measure the volatge out at the LM35 i get 0.250 wich is about 25C +- the after the amp i get 1.167 at the input of the adc wich is displayed as about 25 on the lcd. now if i put my fingers on the LM35 for a little while and re-measure the LM35 is now at 0.3v wich is about 30C the input to the adc is still about 1.167 or there about but no where near 1.5v that it should be Pin configs for 741 1 Offset null 1 2 Inverting input 3 Non-inverting input 4 VCC- 5 Offset null 2 6 Output 7 VCC+ 8 NC i have the 741 connected like this 1 NC 2 Connected as in the diagram below 3 5v 4 gnd 5 NC 6 Connected as in the diagram below 7 5v 8 NC sorry about the criss/crossed lines but i did it in paint and quickly Any suggestions ? i dont know what i am doing wrong ok cpemma in this try i used as u can see in the pic 100ohm and a 1kVAR and just ajusted it until i got what i wanted. I will keep those resistor values in mind. Ok i will try buy a LM358 or LM324 tomorrow and retry. But as i understand is the circuit above correct ? Gonna go hunt down datasheets for those 2 IC now and see what they say. Edit found the 2 datasheets gonna see if i can get hold of a LM358 seems like the best choice ... dont need 4 .. just hope i can find a store that sells them /Edit
Well, I just went through the math on the circuit you just posted, and it's definitely not correct (assuming I did it right). When I wound up with a quadratic equation to find the right value for the pot to get a gain of 5, I thought "this isn't good". Then I did the quadratic formula and found two imaginary roots, which definitely isn't good. The other is also wrong, as you'll get a negative gain. This should be the correct way to get a gain of 5: The resistor values don't matter, just as long as the ratio R2/R1 ~ 4.
You don't want the RHS earthed connection. It has to be a non-inverting amp, otherwise 0.2V in --> -1V out, the op-amp can't go negative with +5V & 0V rails. 1.167V = "about 1.5V" I see whypick1 has the same thing. quadratic equations? Non-Inv, gain N, resistor ratio = N-1:1
Thanks guys i will grab a pack of 4k's tomorrow as well as the LM358. I am sure i have a 1k lying around here. I am going to connect the circuit as whypick1's circuit and hope for the best. 12:14am atm so shops open in a few hours. Hopefully in a few hours i can post success. ooh some thing i dont get whypick1, cpemma say resistor ratio = N-1:1 but in the begining cpemma you said Rf should be 5k if Ri=1k is that because of the two diffrent configurations ? There seem to be a lot of diffrent configurations for a non-inverting amplifier ... so which one should i follow ? I take it for every configuration the resistor values will be diffrent to achive the same again? or have i missed the plot completely Thank you, once again
Bah, I probably did the equations wrong when I did it for Halo's second circuit (even though I know it's a bad idea, I find myself doing equations in notepad quite often). Halo, cpemma was referring to the gain on your circuit, not mine. Here's how mine works: For all of these calculations, we're going to assume that the op-amp is ideal, so the inputs draw no current, no current is lost in the output, and the potential of the inputs are the same. With those conditions set, we can find out what the gain is. For now, let's just deal with this symbolically: Vp = Op-Amp Non-inverting input Vn = Op-Amp Inverting input Vout = Op-amp Output Gain = Vout/Vin Resistors R1 and R2 corresponding to the one's in my diagram. Since our op-amp is ideal, Vp = Vn, which, as Vin is tied directly to Vp, both equal Vin. Now we only have one equation to solve for the gain, which we find using Kirchoff's Current Law: Vin/R1 = (Vout-Vin)/R2 Multiply each side of the equation by the other's numerator to get rid of the fractions: R2*Vin = R1*Vout - R1*Vin Add R1*Vin to both sides: (R1+R2)Vin = R1*Vout Divide both sides by (Vin*R1): (R1+R2)/R1 = Vout/Vin = Gain Now, to get the desired gain of 5, simply put that into the equation: (R1+R2)/R1 = 5 Multiply both sides by R1, then subtract R1 from each side: R2 = 4R1 Voila.
Thank you guys, you are a great help. My electronics is rather bad, but my digits side and programming is the much better side 2:07 am got 2 be up at 8 to go buy that opamp better get some sleep will repost in a few hours as to how things go
I don't think 4k is a standard resistance value. Assuming you've only got E12 values available, try 1.2k and 4k7.
I actually didnt think about that... I will try 1.2k and the 4k7 combination and if i need to try 1.2k and (4k7 series with 100) to get a more = 4. There shouldnt be a problem sticking that resisor in series ?
There shouldn't be a problem putting resistors in series to get a tighter ratio. I just used 1k and 4k to make the math simpler. The important part is that the ratio be 4:1 between R2 and R1.
I eneded up buying 2x LM358 2x LM324 1x IR diode 1x IR Reciver I built the circuit above and at least now i dont get a constant voltage out when i touch the LM35! It now varies when i touch it the temp goes up and when i stop it cools down. Hurray. That 741 is useless just brings about problems . I will take the circuit into an airconditioned room later and see how close it is to the actuall room temperature and then decided if i should add extra resistors. As for the IR diode and IR reciver that has to go into another project that has to do with a beam break. Which i will try later 2 day to get working. Thank you guys so much for helping me with this opamp stuff, you have saved me a great deal.
I know this doesn't really help your current situation, but another option is to use a serial protocol device like the DS1620 or equivalent that do not require analog scaling.
In defense of the legendary 741, I think simply you don't have a high enough operating voltage. It is designed to operate with ±5V bi-polar supply (VCC- = -5V, VCC+ = +5V), I.e. 10V accross the device. A good way is to power it with a single 9V PP6 battery, that always works. Also remeber the output can only go to within 1.4V of VCC+. If you've only got access to a 5V supply you will have to look at modern low-voltage OpAmps. PS: I think your circuit design is fine, off the top of my head!
LM35 WITH LM358P WHYPICK1 WHYPICK1 WHYPICK1 I am stuck in exactly the same problem. WHYPICK1 Circuitory is not visible helppppppppp
