Schematics of delabs

One of the oldest Schematic Archives on the Web. It was in the form of pdf files in the late 90s. Instrumentation and Industrial Designs by delabs and Generic Application Designs.

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Wednesday, December 17, 2014

Battery Backup Supply

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.

From my Power Electronic Circuits

Battery Backup Supply

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.

Tuesday, December 16, 2014

AC conversion and Continuity - DMM

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.

AC conversion and continuity buzzer test

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.

Tuesday, December 02, 2014

Isolated dual power supply from 5V

This is a unregulated supply for low power circuits. You may be able to regulate the outputs with zeners or small regulators like 78L05.

The transformer can be hand wound in a mini ferrite pot core. you can use 2N2222 or any other fast transistor. The transformer should have 1KV isolation. The dot polarity of TR1 should be properly observed, else it may fail to oscillate or give output.

Diode should be fast recovery type, for less than 100mA use 1N4148. transformer, pri-20-20, sec-60-60, a SWG-AWG to suit the current you

design for, any fast switching transistor would work, no regulation, use regulators like 78L12 if you want, circuit like multivibrator used for flashing LED lights.

Isolated dual power supply from 5V

The Source file in CadSoft EAGLE format is here del00010.zip

Friday, November 14, 2014

DMM range and AC-DC mode Logic

This is the third circuit that you will need to build a Simple Benchtop DMM with no Microcontroller.

Here the U1D and U1B 4093 acts like a de-bouncing circuit for the push keys. The 4029 counts up scrolling to binary values 00, 01, 10, 11 for the four ranges.

The binary value of 4029 is decoded to decimal by 4028 in order to light four LEDs which indicates the range or mode on the front panel. When any of these pushbuttons are pressed and held, the nand schmitt 4093 clocks to scroll the range continuously. The binary output of these counters control CMOS switches 4052 which are analog multiplexers.

DMM range and AC-DC mode Logic

The DPM or DVM gives a readout of an Analog Value or process. The Analog reading we obtain from a Circuit Measurement Jig represents some real world parameter.

In a multi-parameter instrument like DMM, the measurement jig functions and the analog signal routing is done by ganged range selection switches.

Read More here DMM range and AC-DC mode Logic

Wednesday, November 12, 2014

Power supply with battery backup - DMM

This is a simple charger circuit which will work for a light load like a DMM, the Battery can be a sealed maintenance free battery of 9V-2AH or better. The circuit will work best if the Unit is powered on many times daily on regular use, else battery will drain down.

The 555 Astable is used to generate a AC signal from which a negative voltage is generated, A 79L05 which is a low power TO92 equivalent of 7905 a negative -5 volts regulator is used as -5 volts load is less. A TO220 7805 is used for the +5V supply.

Power supply with battery backup - DMM

Many dual supplies are derived from one DC Source. A SMPS solution is the best. A Series Regulator is simple to troubleshoot in comparison to Switching Types, there is no EMI-RFI too.

Power supply with battery backup - DMM

This simple circuit is ok only for Low Current gadgets, Whereas SMPS is green and efficient. In SMPS a greater care for Product Safety is required. In a Linear Supply with Step-Down Mains-Frequency Transformer. The Transformer is the only place, where you look into safety the most. In SMPS it is the PCB, the feedback components and also The High-Frequency (200 kHz) Mains Ferrite Transformer.

Tuesday, November 11, 2014

Build a DMM - LM3914 Analog display

This a part of a complete Benchtop DMM that you can build to learn the internals of a Digital MultiMeter. This is oriented towards learning the measurement aspects and the protection of input circuits.

U2A opamp LF353 is used here as a constant current source, R10-D13-D14 is for protection in case you measure voltage in the ohms range. U1 4052 helps digitally select four different currents, the currents pass thru the unknown resistor to be measured and an voltage developed across the resistor is measured.

U2B is a buffer which passes on the voltage measured to U5A for inversion of polarity as current source is a negative current (current sink). U5B amplifies to the level required for the LM3914 display circuit. U7 555 is used as a de-bouncing for switch SW1 so as to advance counter 4029 to change the range of resistance measurement.

Build a DMM - LM3914 Analog display

Use any FET input dual opamp, TLO72 will work but LM358 will cause error as it is transistor input..

D1, D12 and R8 are to ensure that the FET can be turned off, as the opamp swings from +/-3.5V only, with some FET it needs to be tweaked. LF353 is a Wide Bandwidth Dual JFET Input Operational Amplifier.

Build a DMM - LM3914 Analog display

Saturday, October 18, 2014

Presettable Up-Down Counter Timer

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.

Digital Circuits 2 from delabs

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".

Wednesday, October 15, 2014

Music Sound to light converter

This was made by me when i was a kid, it was even published in a magazine, i think it will work, it was used on a very old huge ornamental valve philips stereo system in the remote speakers.

Home Made Circuits

Recently i captured this circuit in eagle to publish online, it seems to have a small design error. It worked well for many years. The step-down transformer is used as a step-up here. The secondary winding was around 50mA so the resistance was protecting the 2N2222. (SL100 was used in the original one). C1 and R3 may need tweaking to get the desired adjustment range in R2.

Music Sound to light converter

The voltage across the speaker is fed to C1-Gnd, which blocks DC component. R1, R2, R3 form a threshold bias for T1. 2N2222 chops the 12V DC in tune with the music.

Basics of Electronics

The chopped DC current flowing in the 12V winding of TR1 is stepped up into AC pulses over 100V, this lights the Neon in flashes synchronizing in real-time with the beats. Known Issues - It does not perform well at low Bass like 20Hz.

Tuesday, October 14, 2014

Simple Water operated relay

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.
Simple Water operated relay

Friday, September 05, 2014

80C39 and MCS48 based Process Controller

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.

 80C39 and MCS48 based Process Controller - CPU

Process Controller code here 80C39 Code for MCS48

The circuit for these are on this page along with other circuits. This may be difficult to put together now. But

the code and hardware is near compatible to 8051 uC. Now there are may types of ADC and uC.

Voltage to Frequency Converter AD Interface is the analog front end of this instrument. Uses a LM331 VCO. The Signal conditioning too is integrated in this circuit.

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.

Friday, August 15, 2014

Digital to Analog Converter with uC Watchdog

This is the continuation of the earlier post. Part of 80C39 based Process Controller. In this schematic you can see the Watchdog and D/A Converter.

80C39 and MCS48 based Process Controller is the main circuit that has the LED 7 segment display for output and push keys for input. The old form of Human Machine Interface - HMI.

Digital to Analog Converter with uC Watchdog

My first observation of a very complex watchdog in action was an Agilent(hp) Benchtop Multimeter based on this 8048 family of 1st generation microcontrollers that did not even have a UART among many things.

At that time CMOS was just making an entry and FLASH memory was unheard of. The UV Eprom was the way firmware was set on these systems. These consumed a lot of power. 80C39 was the CMOS one.

The  4040 counter derives a slow clock from the 7555 timer. The counter has to be reset by firmware by periodically sending a reset pulse on port pin P2.7 to say "Alls Well".

If the firmware or uC "hangs" or due to EMI or Spikes the uC gets into an endless loop. Then the "Alls Well" pulses stop coming. The 4040 keeps counting till Q10 output goes high and resets the uC or can we say Wakes it up rudely.

The D/A converter was used to get the 1-5 V to obtain 4-20 mA control Signal to operate the Actuators like a Motor Drive or Heaters in a Industrial Process control System.

Thursday, August 14, 2014

Regulated High Voltage Power Supply

The Circuit below is a paper design and not tested. It can be used for education and information, this can help you make your own design. Please do not just wire it up and expect it to work.

Regulated High Voltage Power Supply

Now let me see if i can explain the circuit, This is a regulated AC power supply. This circuit uses the Mosfet to turn off when voltage goes beyond a reference point. That means it just chops the Sine wave above a point, that also implies that the output may not be pure sine and may have harmonics. The Transformer if well designed may smoothen the chops. Even a Series Inductor or Resonant Circuits may reduce harmonics.

Regulated High Voltage Power Supply - del20032

The opto coupler 4N50 Provides isolation and good Current Transfer Ratio. That may mean you may not get a shock and that even a small current signal in Opto-LED will give a saturated or Low Impedance in Opto-Transistor. The Mosfet is used like a Impedance Control switch turned On-Off by Opto. The Optocoupler diode is controlled by the Opamps which work Closed loop. The transformer output is compared with reference to drive opto-led.

This Circuit is based on Teledyne Solid State data book application note. They may not be making these parts anymore but they are available from others.

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