Colored light test pen, red and green light multi-functional induction test pen to check on and off breakpoints

A multi-functional test pen, the complete pen body is composed of the PCB itself and is powered by a CR1220 battery. Measurable voltage range <500V. In fact, this thing has the same function as the "color light meter pen" sold online. I am just imitating it.
Safety instructions: R1 and R2 are equipped with 1.2M~10MΩ, 1/4w high-voltage glass glaze resistors. For safety reasons, it is recommended that the parallel value of these two resistors should not be less than 1M. Ordinary color ring resistors do not have enough voltage resistance and may break down and cannot be used.
The smaller the resistance, the more sensitive it is. R2 is larger than R1, otherwise the red light (R1's path) will be diverted too much and become insensitive.
 
The basic feature is that if there is a path from the handle to the pen tip, the green light will be on. If a path is formed from the handle to the pen tip, and the pen tip has a voltage that is tens of volts higher than the handle, then the red and green lights will also be on, and the green light will be brighter than when the pen tip has no voltage. The higher the voltage, the brighter it will be. Flexible use of these two characteristics can make a variety of measurement judgments including but not limited to the following:
 
Battery self-check: The pen does not touch other objects, the human body touches the tip and tail of the pen at the same time, and the human body forms a circuit. If the green light is on, it means the battery is charged. .
 
The basic function is the same as that of an ordinary electric pen. A person stands on the ground and is connected to the ground. He holds the metal part of the pen tail handle (the wider part, the handle in the picture), and uses the pen tip (the narrower part, the testPin in the picture). ) is in contact with the object to be measured.
 
Determine the neutral wire and ground wire (grounded object): Hold the metal part of the pen tail and touch the pen tip to the object. Only the green light is on, indicating that the object is grounded (a loop is formed through the earth and the human body). Pictured is the neutral line and a metal table on the ground.
Determine the live wire (charged object): Hold the metal part of the pen tail and touch the pen tip to the object. The neon bulb will light up, the red and green lights will light up, and the green light will be brighter than the neutral line, indicating that the object is charged. If the battery is not installed or the battery is dead, the light will not light up, but the neon bulb will light up and the object can be found to be charged. If you don't want the neon bulb, you can short-circuit the neon bulb's pad to improve the sensitivity of the red light.
To measure continuity: both ends of the wire touch both ends of the pen at the same time. You can also touch one end of the wire with one hand (please make sure the wire is not electrified first!), hold the metal part of the pen tail with the other hand, and touch the tip of the pen to the other end of the wire. The green light is on, indicating that the wire is connected.
Non-contact induction electricity measurement: Hold the metal part of the pen tail and bring the pen tip close to the object without touching it. If the green light is on, it means the object is charged. It can be used to find the breakpoint of the live wire that is disconnected somewhere: across the wire skin, move the pen tip close to the live wire. When you move to a certain place where the green light goes out or becomes significantly darker, it means that this is the breakpoint of the live wire.
The induction is relatively sensitive and will induce a green light on some things that contain a lot of high-frequency interference but are not actually charged (such as the output of various switching power supplies and chargers). You can dismantle R3 and R4. If you dismantle one, the sensitivity will drop a little. If they are all disassembled, they will be the least sensitive. Replacing Q1~Q3 with transistors with lower amplification can also reduce sensitivity. According to the actual measurement, the sensitivity is available when neither R3 nor R4 is installed, and the brightness of the green light is slightly low. Just one is too high. In the future, it is recommended to try changing to a triode with a lower amplification.
 
It is recommended to use a relatively thin PCB thickness of 1.0, 1.2, etc., to facilitate the situation where the pen tip needs to be inserted into the gap. However, thin PCBs are relatively soft and can easily break accidentally, so you have to make your own choice. It is recommended to wrap a circle of double-sided conductive copper foil tape around the tip of the pen to make contact with the side of the PCB.
 
R8 is used to eliminate the influence of weak leakage on the PCB surface. A weak leakage current from the PCB surface flows into the transistor, causing the green light to light up weakly even when the pen does not touch anything. First clean and dry the PCB surface to reduce the impact of leakage. If the green light still lights up weakly when the pen does not touch any object, you can install R8, but it will reduce the sensitivity of the green light. The value needs to be determined experimentally based on the actual situation. From Try big or small. On the contrary, the value of R8 can be increased. (The metaphysical phenomenon is related to the cleanliness of the surface and the weather and humidity). Even if the debugging results confirm that R8 is not installed, this phenomenon does not occur, and the R8 pad cannot be deleted from the design, because the weak surface leakage current between the pads actually still acts as a resistor. According to actual measurements, even after adding the R8 pad Without R8 installed, this phenomenon almost no longer occurs, so R8 is not actually installed.
 
This is also a good example of getting started with analog electronics. You can think about the following, why can only charged objects make the red light turn on (what is the role of Q4?), while the green light will turn on as long as there is a path? Why can induction power measurement be realized (involving [distribution parameters] not mentioned in the book!)?