Use TL431 to make a TTL logic level detection circuit

The author recently saw a comparator circuit and a DC/DC converter circuit made with a TL431 device from the information on the three-terminal adjustable reference voltage source TL43I, and was inspired to design a TTL logic level detection circuit (detection pen) .

The TTL logic level detection circuit composed of TL431 is shown in Figure 1. Its working principle is as follows: V+=5V, when the detection voltage is lower than 2.5V (TTL low level), Vka=5V, vT is cut off, and the LED does not light up; when the detection voltage is higher than 2.5V (TTL high level), Vka=2V , vT is turned on and the LED is on. Generally, the high level is above 3V and the low level is below 1V, so it can be used as TTL logic level detection.

1. Selection of components

The TTL level detector composed of the above circuit can be made into two structural forms, as shown in Figure 2. Figure 2(a) is the circuit installed in a small box or plastic bottle cap; Figure 2(b) is Install the circuit board in a transparent pen holder to make a test pen.

The structure in Figure 2(a) can use general components: TL431 is packaged in TO-92; LED is 3 or 5; VT is 9012; resistor is 1/10-1/4W. The inner hole of the pen holder in Figure 2(b) is small, and the components are of the SMD type. Choose SOT-89 package for TL431, 2SB624 (SOT-23 package) for VT; 0805 resistor; and SMD type for LED.

2. Production of TTL level test pen

1. Preparation

(1) Find a transparent ballpoint pen holder with an inner hole diameter greater than 6mm, and open a through hole 3 in the back cover.

(2) One red and one black multi-stranded flexible wire (400~450mm) each, and weld the corresponding color crocodile clips.

(3) For scrap bicycle bars (length about 100mm), file a groove as shown in Figure 3 and file the other end into a conical shape.

(4) Cut a fiberglass copper-clad plate with a width equal to the inner hole of the pen barrel (can be inserted freely), a length of about 45mm, and a thickness of ~1.5ram (as shown in Figure 4).

(5) Use the "isolated island knife carving method" to carve the printed board (see Figure 4). For the convenience of carving, use jumper wires for the connecting lines (as shown by the dotted line in Figure 4).

(6) Drill holes as shown in Figure 4 and solder a thin layer of solder on each "isolated island".

(7) Tin the jumper wires (0.4-0.5) and the soldering ends of each component (the solder layer must be thin and uniform).

2.Welding

(1) First solder three jumper wires.

(2) Use the tip of a toothpick to dip a little bit of 502 glue onto the back of TL431, and paste TL431 on the printed board (key points: there must be less glue, and do not apply glue on the "island" or welding end, where it is pasted Be sure to be precise). Weld each solder joint after it is firmly adhered. Paste the components and weld in sequence in the same way. Key points: Use a 2-point soldering iron, and use 0.80 rosin wire for better results, and after the soldering is completed The welding quality of each component must be checked: whether there is any virtual welding or "adhesion" at the welding end.

(3) Use hard wires (0.4 to 0.6) to tie the strip and the printed board tightly, and solder them firmly.

(4) Solder the positive and negative soft wires with alligator clips (red wire connected to +, black wire connected to -, see Figure 4). Before welding, the soft wire should be inserted into the back cover of the pen barrel.

3. Check

Check the quality of each solder joint again, and use a 1k three-purpose meter to measure the resistance values ​​of the + end, one end and the detection end: when the red test lead of the three-purpose meter is connected to "+" and the black test lead of the three-purpose meter is connected to "-", The resistance is about 13kO; when the black test lead of the three-purpose meter is connected to "+" and the red test lead of the three-purpose meter is connected to "-", the resistance is infinite; the black test lead of the three-purpose meter is connected to "-" and the red test lead of the three-purpose meter is connected to detection. point, the resistance is about 75k ohms, when the black test lead of the three-purpose meter is connected to the detection point, and the red test lead of the three-purpose meter is connected to "-", the resistance is about 19kΩ.

If the checked resistance value matches the above value, it means the welding is correct. Install the printed board into the pen holder, install the back cover, screw on the nut, and the assembly is completed.