Category: Handy Circuits

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Resetting Arduino via serial line

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Normally Arduino boards are reset by using additional DTR line of serial interface. This becomes a problem when USB-UART adapter doesn’t support DDR line. And you probably read many cases were one or another particular cable won’t work for programming, but can be used for simple serial data transfers. Ralph thought that there should be another solution that would allow using any serial cable for programming. He thought that TXD and RXD lines are always available since they are used for data receive and transmit. So why not to use one of those to reset microcontroller. With three additional discretes he created a simple circuit that would stand between RXD data line and RST pin. This is simply an RC circuit that would discharge cap during some time. So when… Continue reading

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Reducing interfacing pin count for nrf24l01

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Sometimes we get caught in situations when we need direct solutions without figuring things out in more efficient manner. For instance microcontrollers and pin count. When we need more pins, we start looking for bigger MCU even if processing power is enough even if one additional pin would change the situation. So sometimes this is not effective to waste money and even design to get one or two additional I/Os. Some people may use I/O port expanders or shift registers to get more pins. But as Ralph shows we don’t always need head for obvious. There are tons of discrete electronics components around that may save the day. This time he suggests interfacing well known nrf24l01 RF module to small MCU like Attiny85 by using 3 pins instead of 5.… Continue reading

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Half duplex UART from single AVR pin

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Smaller microcontrollers like Attiny84 microcontrollers don’t have UART interface and in many cases you may not need it. But if you will want to have an USART option, you will need to user software USART library or write your own routines. You can find many great software USART libraries for that purpose. As a rule you will have to use two pins to establish communication. But if you are tight on I/Os then you can cheat a little and make it work from single pin. Ralph has been experimenting with simple but smart circuit which allows to perform half duplex UART communications with other systems. The whole trick lies in a small schematic made of diode, transistor and resistor. Diode is only for making one way TX signal path from… Continue reading

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Reusing LiPo power supply for your embedded project

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Reusing LiPo power supply

Every electronics device needs a power supply. In many cases we just go with AA batteries, 9V + regulator, wall mounted adapter and LiPo cells. First ones are obvious enough – easy to obtain and pretty safe. Lithium technology batteries falls in to different category. These cells carry much more capacity than casual batteries, but also are more dangerous and sensitive to too much discharge and overcharge. This is why in almost all cases LiPo batteries come with small PCB attached to them. This circuit at least takes care of disconnecting battery in case of discharge bellow threshold (2.7V). In battery packs this situation is more complex. Proper circuit should monitor individual cells and disconnect whole pack if at least one cell reaches the threshold. But in reality cheap battery… Continue reading

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Using DIP switch to select voltage you need

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In many cases you simply need a voltage source that could be used right away. It may be 5V, 3V or other like. Many battery operated electronics require voltages that are multiply of 1.5V – single cell. So for instance if device requires 4 AA batteries then it total voltage is 6V. Jason is a fan huge of garage sales where he can find lots of ideas and materials for his projects. But he wants to make sure if devices works properly before purchase. Due to variety of battery types he didn’t want to carry a bag of batteries around. So he came up with nice idea p DIP switch controlled power supply. As you can see there is nothing actually new or extraordinary. An LM317 power regulator is used… Continue reading