# Questions tagged [resistor-transistor-logic]

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### How to translate 0V pulldown switch to negative voltage?

I have a circuit with a momentary switch connected between GND and a net with a pullup resistor (switch output is 0V while pressed). This switch signal is then buffered by a flip-flop before feeding ...
• 38
648 views

### What prevents this transistor based discrete logic gate design from having been used?

I am interested in building discrete logic gates as a teaching / learning tool to explore computer fundamentals. The easiest logic gate design I can come up with consists of the following: It is ...
• 121
1 vote
110 views

### Resistor for open-collector transistor

I have an encoder; the circuit is in the image below: The one that I’m interested in is the PNP one. I want to use a 0-5 V digital voltage, the datasheet says this: Applied voltage: 30 VDC max. ...
2k views

### Help me calculate voltage drops through this circuit

Home hobbyist here, just trying to learn. I'm trying to calculate voltage drops across the various components, and I'm stuck on R4. I calculated R1 and R2 no problem (I think), using: ...
• 459
2k views

### How does a transistor inverter work?

When the base of the transistor is LOW, no current can pass through the collector to the emitter and therefore if the current is 0 the voltage is 0 and all current will pass through the resistor that ...
• 57
1 vote
225 views

### My transistor level half-adder works in spice but not in real life

I'm trying to build a transistor level half-adder. I am using each transistor as a NOR gate (see below). simulate this circuit – Schematic created using CircuitLab The half-adder logic circuit ...
• 39
4k views

### Why do CMOS NOT gate designs differ from BJT NOT gate designs?

If you look for an inverter circuit on Google Images, you'll find something that looks like this: This inverter design has a single NPN transistor. However, on Wikipedia's article about transistor ...
• 281
1 vote
469 views

### How does a two input transistor AND gate work?

I'm trying to self study electronics. I started reading a book on digital electronics and I got stuck on the first chapter. It's about the logic gate circuit. There is a two input AND gate circuit ...
• 11
1 vote
126 views

### Strange behavior on diy OR logic gate

I'm trying to create a transistor logic OR gate for a key chain, it was supposed to work with two buttons, but just a light touch on the button contacts light up the led brighter than when pressing ...
1 vote
693 views

### Why do we use resistors(R1 and R2) in transistor NOT gate?

Why do we use R1 and R2? What happens if R1 and R2 are absent?, will the NOT gate still work?
1 vote
187 views

### Do LEDs (of the same type) in parallel consume different current?

In a part of my circuit, there will be three LEDs (of the same type) in parallel. Is it true that they will not automatically get the same current and have the same brightness? Somewhere I read about ...
• 129
87 views

### NPN BJT NOR gate to drive LED. Base resistance problem

Here's a circuit I'm trying to get working : I have two digital signals (B and C in the diagram) coming from a CD40174 flipflop. I use these to light up two LED's. I also use these as the input for ...
• 299
2k views

### Why do TTL integrated circuits have such complicated schematics for logic gates with so many transistors as opposed to RTL?

I'm in the process of building a 4 bit computer out of discrete NPN BJTs and resistors. I'm using RTL, and I've made flip flops, full adders, and demultiplexers, and everything is working fine so far. ...
• 131
225 views

### Linear technology power supply, question about a transistors

On page 12 of this journal there is a power supply design by LT. On page 14 there is a complete schematic. I understand everything but these two parts: There is a negative supply of -5V connected ...
• 535
634 views

### Why is D2 switched off and T4 not in saturation mode in 7408 TTL Gate as shown below?

In the circuit, when both the inputs are high at 5V, than why is D2 switched off? Will it not draw voltage from Vcc? Also, when one of the input is low, why is T4 not running in saturation mode? Will ...
144 views

### Using transistor output as a logic input

I'm trying to use the output of the following transistor circuit as an input for XOR gate. It doesn't seem to work, I'm out of ideas. Any help would be appreciated.
584 views

### Indicating 3V3 and 5V with 2 LEDs

I am building 3V3/5V source for my breadboard. The actual voltage will be selectable by the switch. I would like indication, which voltage is actually selected by lighting green LED for 5V and yellow ...
• 667
7k views

### What are the limitations of using a LDR sensor?

I have to explain limitations for a photoresistor/LDR . I've read the book, and searched online but find nothing. Can I get help with this ? Are things that the sensor can NOT do, which other tools ...
1 vote
272 views

### LED in NOT gate confusion: LED voltage drop vs transistor voltage drop

I saw a schematic for building a NOT gate, it worked, but there's something I don't understand. Here is a pic of what I did (sorry about my finger hiding the push button) If I push the button, the ...
296 views

### Electron flow with pulling resistors

I'm very confused on the idea of pulling resistors. I get that they set the default voltage to a value so that you don't get a floating high or low value, but I don't get the application in TTL. So ...
539 views

### Error Logic Gates [duplicate]

I connect this 5 transistor and,or,nor,nand,inverter respectively into to switches to operate.. I also add Exor and Exnor.. the Exor I made consists of 4 Nand gates based on the nand diagram stated on ...
1 vote
990 views

### LED with a transistor on each side

I'm trying to make a circuit with a transistor on each side of an LED so it only lights up when both transistors are activated. The transistors need to be on either side of the LED because the LED ...
• 113
5k views

### Using 25 IR LEDs with Raspberry Pi and Transistor

Using a Raspberry Pi and 5V I want to use 25 IR LEDs (5mm LED, 940nm wavelength, 20 degree beam width, 100 mA continuous, 1000 mA pulse, Approx 1.6V forward voltage; http://www.exp-tech.de/super-...
• 103
351 views

### The resistor's power consumption of 1 ohm in watts

I ran into a old-midterm exam question that calculate the one-ohm resistor's power consumption in the following diagram in watts? I need some solution or hints. I try to solve it but I think the ...
1 vote
1k views

### What is the best transistor for a large RTL (resistor transistor logic) project?

I've been reading up on older computers like the PDP-1 and TX-0. I'm fascinated with them and love the idea of making a CPU of my own using discrete resistors and transistors. Keep in mind, this is ...
• 675
352 views

### Car Gear Indicator [closed]

I'm new in here and I'm very new to electronics and have a very little knowledge about it. I'm trying to do a project in where I have a car with manual gears 1,2,3 & 4. Each will be represented by ...
• 11
4k views

### How do I invert a signal without deforming it, and without using a logic gate?

I have an IC which outputs pulses over an open collector pin. I need both these pulses and their inverses in my circuit. I want to do the inversion by using as few as possible transistors and without ...
• 14.1k
1 vote
391 views

### NAND gate from BJTs: interfacing +/- 12v op-amp and 5v logic: destroy BJT with excessive Vebo?

I want to switch on a relay when both a 5v logic input is high and a (LM741 used as a) comparator is around +10v, and the circuit I planned to use is below. The problem I see with the circuit is when ...
• 5,114
11k views

### Why do optocouplers require a resistor at the collector?

In this circuit, if the resistor in the collector R1 is 0 Ohms, the circuit doesn't work. That is, the output will be 12V DC signal as it is from the battery. And if R1 is non-zero, then the circuit ...
• 691
12k views

### How to combine multiple transistor logic gates without gigantic voltage-drop?

In follow-up to my previous question: Resistor values in transistor logic gates I've breadboarded all common types of transistor logic gates: XNOR, ...
• 309
17k views

### Resistor values in transistor logic gates

I breadboarded some AND, OR, NAND and NOR logic gates (...
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