Building a hygrometer with a HS1101


The next sensor I wanted to add to my home was a set of hygrometers. Specifically I wanted an exterior one, and a matching interior one. This would be useful as we have evaporative cooling, and if the humidity level outside is already high, then it doesn’t make a lot of sense to put extra water into the air. Worse than that, it can also damage my books and make the house really clammy. So, adding some sensors was the first step in some form of alerting.

I picked up two HS1101s from ebay quite cheaply (about $4 each IIRC). These devices are capacitors whose capacitance varies proportionally with relative humidity. You also need to provide a temperature at the sensor to correct the value, although the correction is pretty minor so I guess you could skip this if you really wanted to cut costs. Given I have plenty of code for Dallas 1820s now, I just dropped one of those onto the board too.

I just used the circuit from the data sheet for my design, with a few simple tweaks (like the DS1820). Here’s my surprisingly unprofessional circuit diagram:

The DS1820 stuff that’s not on the data sheet is in red. When built, it looks like this (note the crazy amount of jumper wire):

This gives us temperature on a 1-Wire pin, and an oscillator on another pin which relates to the current humidity. You’ll notice that my circuit has some extra wires, that’s because I power down the 555 / HS1101 when I’m not taking a sample. I do this because Peter H. Anderson suggested that noise would be a problem otherwise. This circuit was actually quite hard to build and get working. There are a few reasons for that:

  • The large number of jumpers on the prototype PCB.
  • The lack of documentation from other arduino hackers (with the notable exception of the rather good Peter H. Anderson page).
  • The HS1101 data sheet forgets to mention that connecting pins 1 and 8 on the 555 is assumed knowledge.
  • The values for R1 and R2 vary depending on what model 555 you are using, and are crazily specific. For the LMC555 that I used, R1 is 1238K and R2 is 562K. I got close to these values, but not exact and it did seem to affect accuracy.
  • You must use a CMOS 555. That’s buried in a six word sentence in the middle of a page on the data sheet, and I didn’t notice it for a while. With a NMOS 555, you get effectively random numbers out of the circuit. Worse than that, CMOS 555s are actually a little hard to find, and I had to get mine from Farnell.
  • I attempted to calibrate with the government weather data from the next suburb over. Unfortunately, as best as I can tell, that data is wrong. It claims that its currently as humid here as it is in Cairns in the wet season, which I deny. Calibration is an ongoing issue for me, although I have some ideas on how to progress there. It might also not matter, as I am building an identical sensor for inside the house and as long as they are both equally wrong I can still detect the “turn off the water to the evap” state that I want to.

Here’s a comparison between my data for today and the weather service’s:

The code to run the HS1101 is relatively simple:

// Temperature and humidity sensor node. Based on the FridgeControlWeb project of mine
// as well as

#include <enc28j60.h>
#include <etherShield.h>
#include <ip_arp_udp_tcp.h>
#include <ip_config.h>
#include <net.h>
#include <websrv_help_functions.h>
#include <avr/io.h>
#include <math.h>

#include <OneWire.h>
#include <DallasTemperature.h>

#define ONEWIRE 3
#define HS1101DATA 5
#define HS1101POWER 7

int count_transitions(int ms);

// How long between measurement cycles
#define SLEEP_SEC 10

OneWire oneWire(ONEWIRE);
DallasTemperature sensors(&oneWire);

unsigned long last_checked = 0, this_check = 0;

// Web server setup
#define MYWWWPORT 80
#define BUFFER_SIZE 550
#define ERROR_500 "HTTP/1.0 500 Error\r\nContent-Type: text/html\r\n\r\n<h1>500 Error</h1>"

static uint8_t mymac[6] = {0x54, 0x55, 0x58, 0x10, 0x00, 0x25};
static uint8_t myip[4] = {192, 168, 1, 252};
static uint8_t buf[BUFFER_SIZE + 1];
char data[BUFFER_SIZE + 1];

// The ethernet shield
EtherShield es = EtherShield();

uint16_t http200ok(void)
  return(es.ES_fill_tcp_data_p(buf, 0, PSTR("HTTP/1.0 200 OK\r\nContent-Type: text/html\r\n"
                                            "Pragma: no-cache\r\n\r\n")));

// prepare the webpage by writing the data to the tcp send buffer
uint16_t print_webpage(uint8_t *buf)
  uint16_t plen;
  plen = http200ok();
  plen = es.ES_fill_tcp_data_p(buf, plen, PSTR("<html><head><title>Temperature sensor</title>"
  plen = es.ES_fill_tcp_data(buf, plen, data);
  plen = es.ES_fill_tcp_data_p(buf, plen, PSTR("</pre></body></html>"));


// Float support is hard on arduinos
// ( with tweaks
char *ftoa(char *a, double f, int precision)
  long p[] = {0,10,100,1000,10000,100000,1000000,10000000,100000000};
  char *ret = a;
  long heiltal = (long)f;

  itoa(heiltal, a, 10);
  while (*a != '\0') a++;
  *a++ = '.';
  long desimal = abs((long)((f - heiltal) * p[precision]));
  itoa(desimal, a, 10);
  return ret;

#define cbi(sfr, bit) (_SFR_BYTE(sfr) &= ~_BV(bit))
#define sbi(sfr, bit) (_SFR_BYTE(sfr) |= _BV(bit))

int count_transitions(int ms)
     // configure Counter 1
     cbi(TCCR1A, WGM11);
     cbi(TCCR1A, WGM10);

     cbi(TCCR1B, WGM12);  // WGM12::WGM10 000 - Normal mode

     sbi(TCCR1B, CS12);   // CS12::CS10 111 - External clock, count on rising edge.
     sbi(TCCR1B, CS11);
     sbi(TCCR1B, CS10);

     TCNT1 = 0x0000;      // note that TCNT1 is 16-bits
     // not sure if should turn off the counter

void setup()   {
  // initialize the digital pin as an output:

  es.ES_init_ip_arp_udp_tcp(mymac, myip, MYWWWPORT);

  pinMode(HS1101POWER, OUTPUT);

void loop()
  int i, j, data_inset;
  char float_conv[10];
  float t;
  DeviceAddress addr;
  uint16_t plen, dat_p;
  float relhum_raw, relhum_corrected;
  int relhum_count;

  // Read temperatures, we dump the state to a buffer so we can serve it
  this_check = millis();
  if(this_check > last_checked + SLEEP_SEC * 1000)
    data_inset = 0;
    sprintf(data + data_inset, "Sensor count: %d\n",
    data_inset = strlen(data);

    for(i = 0; i < sensors.getDeviceCount(); i++)
      t = sensors.getTempCByIndex(i);
      sensors.getAddress(addr, i);

      data[data_inset++] = 't';
      for (j = 0; j < 8; j++)
        sprintf(data + data_inset, "%02x", addr[j]);
        data_inset += 2;

      sprintf(data + data_inset, ": %s\n", ftoa(float_conv, t, 2));
      data_inset = strlen(data);

    // Power up the 555 / HS1101, and take a measurement. Power it down again afterwards.
    digitalWrite(HS1101POWER, HIGH);
    relhum_count = count_transitions(1000);
    digitalWrite(HS1101POWER, LOW);
    sprintf(data + data_inset, "HS1101 cycles: %d\n", relhum_count);
    data_inset = strlen(data);

    relhum_raw = 557.7 - 0.0759 * relhum_count;
    sprintf(data + data_inset, "Raw humidity: %s\n", ftoa(float_conv, relhum_raw, 2));
    data_inset = strlen(data);

    relhum_corrected = (1.0 + 0.001 * (t - 25.00)) * relhum_raw;
    sprintf(data + data_inset, "Corrected humidity: %s\n",
            ftoa(float_conv, relhum_corrected, 2));


    last_checked = this_check;

  // Handle network packets
  dat_p = es.ES_packetloop_icmp_tcp(buf, es.ES_enc28j60PacketReceive(BUFFER_SIZE, buf));
  if(dat_p != 0)
    if (strncmp("GET ", (char *)&(buf[dat_p]), 4) != 0){
      // head, post and other methods:
      dat_p = http200ok();
      dat_p = es.ES_fill_tcp_data_p(buf, dat_p, PSTR("<h1>200 OK</h1>"));

    // just one web page in the "root directory" of the web server
    else if (strncmp("/ ", (char *)&(buf[dat_p+4]), 2) == 0){
      dat_p = print_webpage(buf);
      Serial.println("Served temperature web page");

      dat_p = es.ES_fill_tcp_data_p(buf, 0, PSTR(ERROR_500));

    es.ES_www_server_reply(buf, dat_p);

As with previous circuits, I’m going to have to thank Doug for hints and tips along the way, as well as letting me steal his entire collection of 8 pin DIP sockets.