U1A LF353 opamp turns on a buzzer when the voltage at Test + falls very low indicating a short between Test + and Test - or a resistor less than 5 ohms. The R9-R14 divider determines the minimum voltage that can be at Test + for buzzer to just turn on.
The Opamp here is a comparator and R10 10 Meg gives a very small hysteresis so that there is no oscillation at threshold levels, that is when both inputs at same levels.The Zener is for protection and R6 to limit current.
U2A and U1B opamps form a precision rectifier, note that this is not a true RMS rectifier circuit for that see some devices from Analog Devices. When you need to rectify a 200mV AC signal you cannot use a diode like 1N4148 as the diode turns on at 700mV so this circuit is used.
The AC signal measurement is best to do with a True RMS convertor. This Type is able to quantify the Heating power of a AC voltage, includes all harmonics and Noise too.
This is a 9V power supply which will work even on power failure. It uses a rechargeable battery and regulators. A transformer with 15-0-15 AC volts output is required.
In the first regulator U1 the output is lifted up by 1.4V and in the second regulator U2 by a resistor divider. In the second regulator the voltage across resistor R3 is 5V, so the current is 5V / 1K = 5mA this adds to the quiescent current of 5mA from the regulators ground terminal and flows into the resistors R1 and R2 in parallel which form 404 ohms, 10mA thru 404 ohms is 4V. So the output will be 5 + 4 = 9V. Note that the charge and discharge paths of the battery are separated with diodes.
This was done in my early days, i have upgraded it, it ought to work, reduce the number of transistors to make it less sensitive, also a lower value in place of 10M will reduce its sensitivity, use clamping diodes to protect.
BD139 is used to drive the relay as it has good Ic. So you can even use a low ohm relay. If a Relay resistance is high its quality is higher, its power consumption is less and it needs thinner wire SWG-AWG. T2 and T3 form a darlington pair which drives T1. LED1 shows that the water level has reached the top of tank and also that the Relay is energised.
D1 a freewheeling diode. R3 10M ensures that the high gain input does not float, yet the low leakage current thru the water is not drawn away by the 10M. R2 limits base current in case water is saline.
When i had put the near Obsolete digital circuits online in the late nineties. One person who works in a public institution in the usa, wanted a modification of one of my existing circuits. He had those parts the CD40 Series Logic Chips. He wanted to use only those that he had in his Stock.
I made some modifications and sent it to him, that helped him with his task. These things can be done very easily using the Arduino. One could make a programmable Arduino Timer/Counter with a matching Configuring Software without coding, for such people. Easy and Affordable.
Circuit 1 - Digital Timer Clock With Preset using Thumbwheel switch.
A Thumbwheel Switch has to be used in place of DIP switch shown, just know that 1-2-4-8 nibble (4 bit) should be generated by Thumbwheel switch at preset or jam inputs of 4029.
Use CD4511 if 4513 is not available, but circuit has to be changed a bit around 4511
Circuit 2 - 1 Hz or 1 pps crystal clock using CD4060 and 32768 Hz Crystal.
They have not been tested much... The 4513 control pins 8-4-5-3 connections verify, as i did not get the datasheet.
The circuits will work as the concepts are right, but some tweaks in R C values may be required.
the R C values can only be corrected if you have problem in making it work.
The main problem in the R C values may be related to "the reset at 6 for the tens of seconds and the tens of minutes".
The Early Microcontroller : 80C39 is a CMOS Version of 8048 the one that preceded 8031-8051. MCS48 is the set for that, MCS51 is the current set.
Digital to Analog Converter with uC Watchdog is the front end and an external hardware watchdog of an early kind. These are simple but robust circuits and worked well in noisy environments.
Here is an example project for 80C39-8748-8749 microcontroller. This code was written by me, and it works. But the documentation is not complete or may have errors.
This was supposed to have Analog in and Analog out. Some parts of the circuit may be in the 80C51 page. This has ramp-up and ramp-down settings for the Increment and Decrement buttons. This helps setting setpoint quickly. There is also debouncing for the pushbutton.
