Interactive electronics resource
High / Low Alarm Kit Instructions
Build, understand and experiment with the High / Low Alarm Kit. This resource brings the original kit material into the current MitchElectronics format with the schematic, circuit explanation, component and PCB information, construction guidance, troubleshooting and practical ideas where available.
Introduction
What is the Hi/Low Alarm?
The Hi/Low Alarm Kit is an alarm circuit that sounds a beeper when a measured voltage becomes too small or too large. The detection point for both the low and high voltage levels can be set using the two onboard potentiometers, and the rate of beeping can also be changed with another potentiometer.
This kit uses an LM358 as a dual comparator to detect both low and high conditions, and a 4093 quad NAND Schmitt trigger provides tone generation and gating.
How can the Hi/Low Alarm be used?
This kit has two primary uses, with the first being the detection of dangerous operating conditions, and the second being the detection of sensor readings out of range.
In the case of dangerous operating conditions, this kit can be used to monitor the output voltage of a power supply, and sound an alarm if that voltage becomes too small or too large (such as in battery charging applications).
In the case of sensor readings, this kit can be used to control devices that react to changes in sensor outputs. For example, a water level monitor could use this to sound an alarm if a water tank needs filling or automatically power a pump to refill the tank with water.
Schematic
How does the high/low alarm work?
The Hi/Low Alarm consists of three main circuit sub-blocks; a comparator stage, an RTL NOR gate, and a gated tone generator.
To start, an external voltage is fed into VIN (J2), which passes through a potential divider consisting of R1 and R2. The extremely low value of R1 compared to R2 (100Ω << 100KΩ) means that the voltage across R2 is almost identical to the voltage being fed into VIN. The purpose of R1 is to provide some series limiting the current that could otherwise damage the circuit should a fault occur on the VIN input.
This voltage across R2 (which is the same as the voltage on VIN), is fed into two different comparators, U1A, and U1B, which each have their own separate potentiometer.
U1A is configured as a non-inverting comparator which will output a logical 1 when the voltage present on VIN is larger than the voltage set by the potentiometer RV1. This op-amp indicates a “voltage high” condition, and is also connected to the LED D2.
U1B is configured as an inverting comparator that will output a logical 1 when the voltage present at VIN is smaller than the voltage set by the potentiometer RV2. This op-amp indicates a “voltage low” condition, and is also connected to the LED D1.
Each comparator has an additional pull-down transistor on their outputs, and the combination of R5, Q1, and Q2 makes an RTL NOR gate. Simply put, if any of the comparators outputs a logical 1, the voltage at the input of U2A will be pulled down to ground (i.e., 0V).
If the input to U2A is pulled to ground, the output of U2A will switch to 1 as it is configured as an inverter. The second NAND gate, U2B, is configured as a tone generator with R6, RV3, and C1 (see inverting Schmitt trigger oscillator). If the output of U2A is a logical 1, the tone generator U2B outputs a square wave whose frequency is dependent on the resistance of RV3.
Finally, the output of the tone generator is connected to another NAND gate configured as an inverter (U2C), and this, in turn, powers a buzzer controlled by Q3. The diode D1 prevents the buzzer from generating large negative voltages that can damage the transistor Q3.
Project ideas
Power Supply Monitor
The Hi/Low Alarm kit can be used to monitor the voltage output of a power supply and warn nearby users if the power supply voltage drifts too much. This can be extremely important in applications where circuits cannot tolerate large changes in supply voltage (such as microcontrollers and microprocessors), as well as circuits that are under test (such as in laboratory conditions).
Water Tank Controller
The Hi/Low Alarm kit can be used to monitor the voltage output of a power supply and warn nearby users if the power supply voltage drifts too much. This can be extremely important in applications where circuits cannot tolerate large changes in supply voltage (such as microcontrollers and microprocessors), as well as circuits that are under test (such as in laboratory conditions).
Try it yourself
Circuit simulation
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What you need
Component List
| Component | Quantity | PCB Reference |
|---|---|---|
| 8 DIP Socket | 1 | U1 |
| 14 DIP Socket | 1 | U2 |
| LM358 | 1 | U1 |
| 4093 | 1 | U2 |
| 100Ω Resistor | 2 | R1, R6 |
| 1KΩ Resistor | 4 | R5, R7, R8, R9 |
| 10KΩ Resistor | 2 | R3, R4 |
| 100KΩ Resistor | 1 | R2 |
| 10KΩ Potentiometer | 3 | RV1, RV2, RV3 |
| 100nF Capacitor | 2 | C2, C3 |
| 100µF Capacitor | 1 | C1 |
| Red LED | 2 | D2, D3 |
| 1N5817 | 1 | D1 |
| 2N3904 | 3 | Q1, Q2, Q3 |
| Buzzer | 1 | BZ1 |
| Red Wire | 1 | J1 |
| Green Wire | 1 | J2 |
| Black Wire | 2 | J1, J2 |
| High/Low Alarm PCB | 2 | J1, J2 |
Inspect the board
Interactive BOM
Board reference
PCB & assembly
The PCB silkscreen and component references should be checked against the component list before soldering each part.
Before applying power
- Check every component against its PCB reference.
- Confirm the orientation of all polarised components and ICs.
- Inspect for solder bridges, unsoldered pads and clipped leads that could cause a short.
- Check that no loose wire or solder debris remains on the board.
Build with confidence
Construction tips
Recommended build order
A reliable way to assemble High / Low Alarm Kit is to work from the lowest-profile components to the tallest. This keeps the PCB easy to access while you solder.
- Fit resistors, links and other low-profile components first.
- Fit small capacitors and diodes, checking polarity where applicable.
- Fit IC sockets and small semiconductors, observing the orientation markings.
- Fit larger capacitors, potentiometers, switches and other controls.
- Fit LEDs, connectors and the remaining taller components.
- Insert socketed ICs only after soldering around the socket is complete.
- Inspect every joint and check for solder bridges before applying power.
Electronics construction guidance
If you are new to kit construction, use the Soldering Guide alongside these instructions. Identify each component before fitting it and compare its reference with the component list and PCB silkscreen.
When it does not work
Troubleshooting
Nothing happens when power is applied
- Confirm the supply is connected to the correct input and with the correct polarity.
- Check that ICs, diodes, LEDs, transistors and electrolytic capacitors are fitted in the correct orientation.
- Compare component values and positions against the component list and PCB reference.
- Inspect for missed joints, dry joints and accidental solder bridges.
The circuit powers up but does not behave as expected
This kit is intended for upper/lower threshold alarm. If the output is stuck, unstable or outside the expected behaviour, use the schematic to trace the circuit a stage at a time rather than replacing several parts at once.
Check the components around the part of the circuit responsible for the output or timing first. A misplaced resistor, reversed semiconductor or poor connection can allow a circuit to power up while preventing it from operating correctly.
The circuit works intermittently
Intermittent behaviour is often caused by a marginal solder joint, a loose connector or a component lead that has not been fully soldered. Gently inspect the board with power removed and reflow any joint that looks dull, cracked or incomplete.
If the fault remains, compare the assembled board with the schematic and PCB reference one connection at a time.
Ready to test and experiment?
The separate Hi and Low outputs allow for connecting other circuits Consider using a relay driver to automatically control devices
Once the board is working, compare its behaviour with the schematic and the explanation above. Try changing only one input, control or permitted component value at a time so you can clearly see what effect that change has on the circuit.