How can a switching circuit latch on and stay on, and how does feedback give a clean switching action?
Latching switches and feedback: using a relay or positive feedback to latch an output on, the need for a reset, and how feedback gives a snap (Schmitt) action that avoids chatter.
An Eduqas GCSE Electronics answer on latching and feedback in switching circuits: using a relay or positive feedback to hold an output on once triggered, the need for a reset, and how positive feedback produces a clean snap (Schmitt) switching action that avoids chatter near the threshold.
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What this dot point is asking
Eduqas wants you to explain how a switching circuit can latch (turn on and stay on after the trigger has gone), why a reset is then needed, and how positive feedback gives a clean snap (Schmitt) action that stops the output chattering when the input hovers near the threshold. These ideas make sensing and alarm systems reliable.
The answer
Latching an output on
The need for a reset
Positive feedback and the snap action
Hysteresis: avoiding chatter
Examples in context
Latching and feedback make real systems dependable. A burglar or fire alarm latches on so it keeps sounding until staff reset it; a fault lamp latches so a brief glitch is not missed. The Schmitt snap action cleans up the output of any sensor that changes slowly, such as a light-operated switch at dusk or a temperature controller near its set point, and it reappears as the bistable latch and flip-flop in the sequential-logic module, where positive feedback is used to store a bit.
Try this
Q1. State what positive feedback does to a switching action. [1 mark]
- Cue. It reinforces the change, making the output snap cleanly (and can latch it on).
Q2. State why a latched alarm needs a reset switch. [1 mark]
- Cue. The feedback holds the output on after the trigger, so a reset is needed to break the holding path and turn it off.
Q3. State the name for the gap between the two switching levels of a snap-action (Schmitt) circuit and what it prevents. [2 marks]
- Cue. Hysteresis; it prevents chatter (rapid on-off flicker) when a noisy input hovers near the threshold.
Exam-style practice questions
Practice questions written in the style of WJEC Eduqas exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Eduqas 20204 marksA burglar alarm must sound continuously once a sensor is triggered, even if the intruder leaves the beam. Explain how a latching circuit achieves this and what is needed to switch it off.Show worked answer →
Latching action (up to 3 marks): when the sensor triggers, the output turns on (for example a transistor switching a relay or buzzer). A latch feeds part of the output back to keep the input active, so the output holds itself on even after the original trigger has gone. With a relay latch, a spare set of contacts (or feedback to the transistor base) keeps the coil energised.
Reset (1 mark): the circuit stays on until it is reset, typically by a manual reset switch that briefly breaks the holding path (or removes power), allowing the output to fall back to off.
Markers reward the feedback holding the output on after the trigger ends, and the need for a manual reset to clear the latch.
Eduqas 20224 marksExplain why a comparator driven by a slowly changing sensor voltage may switch its output on and off rapidly (chatter) near the threshold, and how positive feedback (a snap action) prevents this.Show worked answer →
The problem (up to 2 marks): near the threshold the sensor voltage may hover around the reference, with small noise pushing it just above and just below, so the output flips rapidly between high and low (chatter), which is unreliable and can buzz the load.
The cure (up to 2 marks): adding positive feedback gives the circuit two switching levels (an upper and a lower threshold), so once it switches it will not switch back until the input changes by a larger amount. This snap (Schmitt) action means small noise no longer crosses both levels, so the output switches cleanly once.
Markers reward the noise-near-threshold cause of chatter, and positive feedback creating two separated thresholds (hysteresis) so the output switches once and cleanly.
Related dot points
- Comparators: comparing two voltages, the reference set by a potential divider, the digital output, and using a comparator to make a sensing system switch at a threshold.
An Eduqas GCSE Electronics answer on comparators: how a comparator compares two input voltages and switches its output high or low, setting the reference with a potential divider, the digital nature of the output, and combining a sensor divider with a comparator to switch a circuit at a chosen threshold.
- Transistor switching: the bipolar transistor as a switch, cut-off and saturation, current gain, and choosing the base resistor to drive a load.
An Eduqas GCSE Electronics answer on using a bipolar transistor as a switch: the cut-off and saturation states, current gain relating collector and base current, and choosing the base resistor so a small input current turns on a larger load current.
- Flip-flops and latches: storing one bit, the difference between sequential and combinational logic, the D-type flip-flop, and edge triggering by a clock.
An Eduqas GCSE Electronics answer on flip-flops and latches: how a flip-flop stores a single bit, the difference between sequential and combinational logic, the D-type flip-flop and how it captures its input on a clock edge, and using flip-flops as memory and to divide frequency.
- Operational amplifiers: the op-amp as a high-gain amplifier, negative feedback, and the inverting and non-inverting amplifier gains.
An Eduqas GCSE Electronics answer on operational amplifiers: the op-amp as a very high-gain difference amplifier, how negative feedback sets a stable gain, the inverting and non-inverting amplifier configurations and their gain equations, and the voltage follower as a buffer.
Sources & how we know this
- WJEC Eduqas GCSE (9-1) Electronics specification (C490) — WJEC Eduqas (2017)