Simple Latch Circuit using Transistors

This article shows you how to build a tiny electronic switch that remembers it is turned on even when you take away the signal.

It uses transistors and other basic parts, making it cheap and easy to build.

You give it a quick signal to turn on and it stays on until you turn it off with another circuit not shown in this guide.

This kind of switch is useful for many things in electronics, like keeping lights on or off.

What is a Transistor Latch Circuit:

A Transistor Latch Circuit is an electronic circuit that uses two transistors which gets latched in response to an external momentary positive signal at the input.

This latch can be broken either by grounding the transistor base or by switching power OFF and than ON.

Circuit Working:

Parts List:

The depicted latch circuit utilizes transistors T1 and T2 in a configuration where T2 mirrors the behavior of T1 either conducting or ceasing conduction based on the input signal received.

T2 also serves as a buffer enhancing responsiveness to even small signals.

When a small positive signal is applied to T1s input, T1 conducts pulling the base of T2 to ground.

Subsequently, T2 starts conducting with a negative bias generated by T1s conduction.

It is important to note the NPN nature of T1, responding to positive signals while T2, a PNP device, responds to the negative potential from T1s conduction.

Feedback Mechanism:

The incorporation of a feedback voltage through resistor R3 plays a pivotal role in the circuit enabling the rapid latching or freezing of the output with a constant positive supply.

This feedback occurs when T2 follows T1, and R3 connects the collector of T2 back to the base of T1.

This feedback loop essentially ensures that T1 conducts forever after the initial trigger creating a latched state.

Capacitor C1 safeguards the circuit from false triggers caused by stray pick ups or switch ON transients.

Restoring the Circuit:

To restore the circuit to its initial state, power can be restarted or the base of T1 can be grounded using a push button arrangement.

This flexibility allows for easy control and manipulation of the latch circuit.

The presented transistor latch circuit finds applicability in various security and alarm systems where a reliable and latched output is crucial.

Formulas:

The transistor biasing in the circuit can be calculated using the following formulas:

1. Base Emitter Voltage VBE = 0.7V

• The voltage drop across a silicon transistors base emitter junction in forward active mode is approximated by this formula.
• Put more simply, there is a typical voltage drop of about 0.7 volts across this junction when a voltage is provided to make the base positive relative to the emitter (forward bias).
• In order to activate the transistor and permit current to pass through it, this voltage drop is necessary.
• It is crucial to remember that 0.7V is only an estimate, and the real Vbe may differ slightly based on the type of transistor, the manufacturing process, and the temperature.

2. Collector Current (IC) = (β + 1) times Base Current (IB)

• An approximate relationship between a bipolar junction transistors base current (Ib) and collector current (Ic) may be found in this formula (BJT).
• The current gain of a transistor is represented by the parameter β (beta).
• The amount that the base current is amplified to create the collector current is essentially what it indicates.
• Stated differently, a β of 100 indicates that a base current of 1 mA will provide a collector current of about 100 mA (1 mA x 100).
• Because a little current also exits the base terminal itself, the phrase (β + 1) is employed.
• This is frequently disregarded in simple biasing computations since it is usually substantially lower than the collector current.
• However, this term is added for more accurate estimates.

3. IC = β times IB

• This formula, which ignores the previously described little base current, provides a more precise connection between the collector current and the base current.
• Based on the current gain (β) of the transistor, it directly connects changes in base current to changes in collector current.

Selecting the Appropriate Formula:

For silicon transistors, the first formula (Vbe = 0.7V) is a constant that is used to calculate the voltage needed for forward biasing.

Depending on the required degree of precision, the second or third collector current formula should be used.

For the majority of biasing computations, the second formula (Ic = (β + 1) * IB) provides a reliable approximation.

Although it is more accurate, the third formula (Ic = β * IB) ignores a little current component.

Recall that knowledge of transistor biasing begins with these formulae.

For a resilient design in actual circuits, other considerations like as temperature changes and component tolerances may need to be taken into account.

How to Build:

Details of constructing the simple transistor latch circuit are as follows:

Identify and Assemble Components:

• Gather all the required components
• NPN and PNP transistors T1 and T2, resistors R1, R2, R3 capacitor C1 and a power supply.
• Ensure that the transistor types and resistor values conform to the design specifications.

Connect Transistors T1 and T2:

• Connect the emitters of T1 and T2 to the ground.
• Connect the collector of T1 to the positive supply voltage.
• Connect the collector of T2 to the negative supply voltage.
• Ensure proper biasing by connecting the base of T1 to the input signal source.

Configure Resistors R1, R2 and R3:

• Connect resistor R1 between the base of T1 and the positive supply voltage.
• Connect resistor R2 between the base of T2 and the emitter of T1.
• Connect resistor R3 between the collector of T2 and the base of T1.

Add Capacitor C1:

• Integrate capacitor C1 in parallel with resistor R1.
• This combination helps prevent false triggers and transient effects during circuit activation.

Power Supply Connection:

• Connect the power supply to the appropriate terminals ensuring that the voltage levels are within the specified operating range for the transistors.

Testing and Adjustment:

• Apply a small positive signal at the input of T1 to initiate the latch.
• Observe the behavior of the circuit.
• If needed adjust resistor values or make modifications based on testing results to achieve the desired performance.

Conclusion:

With these steps, you can construct a transistor latch circuit that effectively latches onto a high output state in response to a momentary input signal.

Adjustments can be made based on specific application requirements and thorough testing is recommended to ensure optimal performance.

References:

Why do all transistor latch circuits (that ive seen) use two transistors?