74LS08 IC is a member of 74XXYY IC series. There are four AND gates in the chip and each gate have two inputs, hence the name. 74LS SN74LS08NSR. ACTIVE. SO. NS. Green (RoHS. & no Sb/Br). CU NIPDAU. LevelC-UNLIM. 0 to 74LS SN74S08D. ACTIVE. Note The ”Absolute Maximum Ratings” are those values beyond which the safety of the device cannot be guaran- teed The device should not be operated at.
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This LED is connected to detect the state of output. The inputs with both switches unpressed allow the pins to float around, depending on the way the IC is set-up internally. I have tried switching to a 9v battery and nothing changes. They are notorious for that particular mishap. Something similar should be done to the right button.
It is currently powered by a 5v 1. These two inputs are connected to buttons to change the logic of inputs. Because total VCC appears across transistors the drop across resistor R1 will be zero.
Output of the AND gate is the voltage across resistor R1. Sara Heart 1 6. After verifying the three states, you can tell that we have satisfied the above truth table. It is really popular and is available 74os08.
Both transistors will be ON and voltage across both of them will be zero. Email Required, but never shown. It might be oscillating as well.
digital logic – 74LS08 AND gate always outputting high? – Electrical Engineering Stack Exchange
Many pins left floating. Looking at the image I agree. If you choose the wrong pair, you end up with an always on switch. With the LS you need to sink 1. In that case, the left button should have a red 74la08 on the lower left pin. For realizing the above truth table let us take a simple AND gate application circuit as shown below.
Pressing the buttons does nothing. The chip is used in systems where high 74,s08 AND operation is needed. You could try adding a pull down resistor k, K, ohm, doesn’t much matter your inputs may be floating high.
Circuito integrado 7408
With that the drop across resistor R1 will be zero. The description for each pin is given below. Sign up using Email and Password. If you want to absolutely avoid learning the pin out of these push buttons, always connect them in diagonal.
TL — Programmable Reference Voltage. In this state the current flow through base of both transistors will be zero. As you can see the LED is on by default despite neither button being pressed. The 5K1 pull-up resistor ensures that the pin is seen as High when the switch is not pressed. Simon Marcoux 1, 2 Because output is nothing but voltage across resistor R1 it will be LOW. There are four legs connected together in pair of two. Then if your IC is good your switches are mis-wired, I have done it myself with that type of switch.
The inputs of the 74LS family and other TTL versions source significant current and will appear 74sl08 a logic High unless firmly pulled to Ground – your k resistors won’t help. The four AND gates in the chip mentioned earlier are connected internally as shown in diagram below. Sign up or log in Sign up using 74la08. I am new to electrical engineering still so forgive 74lx08 if I am making a stupid mistake. When both buttons are pressed. Home Questions Tags Users Unanswered.
You might be using push buttons that are connected in such a way that the legs are shorted connected together. For better understanding the internal working let us consider the simplified internal circuit of AND gate as shown below. Sign up using Facebook. Submitted by admin on 6 April I think that you were dead on in the first place with your answer SaraHeart.
When any one of the buttons is pressed. The two inputs of AND gate are driven out from bases of the two transistors. Pull down resistors as suggested in another answermight also a good idea.